EPI 1 Flashcards
what is veterinary epidemiology
The study of the distribution and determinants of health-related states or events in specified population and the application of this study to the control of health problems
what are the two broad types of epidemiology and describe them
1) Descriptive epidemiology
- Documents the distribution of disease in a population and how it varies across animal, place and time
- Allows us to develop hypothesises then go onto the analytic branch
2) Analytic epidemiology
- Tests the relationship between exposure (‘risk factors’) and disease outcomes
why is veterinary epidemiology important
Potential to make substantial impacts on overall health and productivity by targeting interventions at the population level
What are the 3 factors that describe risk in an individual and a population
INDIVIDUAL 1) host 2) pathogen 3) environment POPULATION 1) individual 2) place 3) time
what are individual-level factors that influence patterns of disease in a population
- Age structure of population - different risk factors
- Distribution of genotypes
- Immunity of population
- Population dynamics
what are the place (spatial) factors that influence patterns of disease in a population
- Proximity to pollutants
- Proximity to infectious agents
- Proximity to disease risks
what are the 3 different types of temporal factors influencing the pattern of disease in a population
1) Subject-referent time
○ Number of days since calving
○ Examples: milk fever in dairy cattle, gestational diabetes in human
2) Calendar time
○ Exact calendar date
○ Examples: influenza in humans, bovine ephemeral fever in cattle - often vary seasonally
3) Epidemic curves are useful way to visualise the temporal pattern of disease
○ When things vary overtime
What are the 4 temporal patterns of diseases and describe
1) Endemic - disease occurs at expected frequency
2) Epidemic - disease occurs at greater than expected frequency
3) Pandemic - huge epidemic (international)
4) Sporadic - single case or clusters of cases
Endemic and sporadic patterns of disease features and examples
Endemic - Disease occurs at expected frequency - Disease present in population or region at all times - Level of disease usually low and predictable - Examples: lameness in dairy cattle Sporadic - Single case or cluster of cases - Infrequent disease occurrence - Irregular unpredictable - Examples - food poisoning
What are the two types of epidemics and define
1) common source
- subjects are exposed to a common noxious influence
2) propagated epidemics
- occurs when the agent is transmitted through the population from host to host
what are the two types of common source epidemics and describe and examples
1) common point source
○ Group is exposed over a relatively short period then disease cases will emerge over one incubation period
○ Curve rises rapidly and contains a definite peck at the top, followed by a gradual decline
○ Example: foodborne disease outbreaks - salmonellosis
2) Common continuous source epidemics
○ Group is exposed continuously and cases emerge over more than one incubation period
○ Curve rises rapidly, no definite peck, plateau of case numbers over time
○ Example: anthrax in cattle
why measure health?
quantify the occurrence of disease
- to determine the importance of different diseases, set priorities, implement control activities, compare level of disease among groups of individuals
define closed population
no additions or removals during a defined follow-up period
define open population
- individuals enter (e.g. births and purchases) and individuals leave (e.g. sales and deaths) during the follow-up period
- The denominator (population at risk) is changing
define morbidity
- describes the amount of disease within a population
- expressed as either incidence or prevalence
What 3 things do you need for a risk (rate) and describe
- a numerator: the number of individuals diseased or dead
- a denominator: the total number of animals (or animal time) in the study group or population; and
- a referent time period - longer time period higher the frequency of the disease
what is the difference between incidence risk and incidence rate and when is each better
rate involves time while risk does not
rate is better for an open population with individuals coming and going however need good record keeping to ensure know when individuals come and leave
define prevalence and the equation
P𝑟𝑒𝑣𝑎𝑙𝑒𝑛𝑐𝑒 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑥𝑖𝑠𝑡𝑖𝑛𝑔 𝑐𝑎𝑠𝑒𝑠 of disease/𝑆𝑖𝑧𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑎𝑡 𝑟𝑖𝑠𝑘
- equals the probability of an individual being diseased at a given point in time
define incidence and list the 2 types
Incidence measures how frequently susceptible individuals become disease cases as they are observed over time
- an incident case occurs when an individual changes from being susceptible to being diseased
1) incidence risk
2) incidence rate
define incidence risk, equation and example
- the proportion of initially susceptible individuals in a population who become cases over a defined follow-up period
- also called cumulative incidence
𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑟𝑖𝑠𝑘 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑐𝑎𝑠𝑒s/𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑠 𝑖𝑛𝑖𝑡𝑖𝑎𝑙𝑙𝑦 𝑎𝑡 𝑟𝑖𝑠𝑘
follow-up period may be fixed eg 5 year incidence risk of arthritis or arbitrarily (lifetime risk)
Incidence rate define, what also called, equation
- Number of new cases of disease that occur per unit of individual time at risk, over a defined follow-up period
- Also called incidence density
Incidence rate = number of incident cases/ (individuals at risk X t)
What is the difference between relative and absolute comparisons and example
Relative comparisons are made by division $5 ÷ $1 = 5 ‘you have five times as much money as me’ [$ units cancel out]
• Absolute comparisons are made by subtraction $5 – $1 = $4 ‘you have $4 more than me’
When to use a 2 x 2 table
- when comparing two groups of animals with different exposure and disease outcomes
- is there an association between exposure and diseases?
- if so how strong is it?
Once you have used the 2 x 2 table to find R+, R- and RT what are the 3 measures of association and types within
(1) measures strength
1. incidence ratio 2. odds ratio
(2) measures of effect
1. attributable risk
(3) measures of total effect
1. population attributable fraction
What are the two measures of strength and are they relative or absolute
1) risk ratio = relative risk/comparison
2) odds ratio = relative risk/comparison
define incidence risk ratio, equation, example of sentence and important number
- the incidence risk of disease in the exposed divided by the incidence risk of disease in the unexposed
- E+/E- = RISK RATIO
the incidence risk of something was x times greater than the incidence risk of other exposure - if the incidence risk (or rate) ratio equals 1
○ then the risk (rate) of disease in the exposed and non-exposed populations are equal - if the incidence risk (or rate) ratio is greater than 1
○ then exposure increases the risk (rate) of disease with greater departures from 1 indicative of a stronger effect
odds ratio define, when to use, when is it similar to risk ratio
- The odds of disease in the exposed divided by the odds of the disease in the unexposed
- Some studies incidence rate can’t be estimated
- when the number of cases of disease are low relative to the number of non-cases (that is, the disease is rare) the odds ratio will be the same as the risk ratio
What is the cross production ratio
OR = cross product ratio = odds ratio OR = Odds exposed/ Odds
Attributable risk where belong, what type of comparison define, equation and how use in sentence
Measures of effect
ABSOLUTE COMPARISON
- the incidence risk of disease in the exposed that is attributable to exposure
AR = RE+ - RE-
the incidence risk of Johne’s in paddock-reared calves attributed to being reared in a paddock was 5.6 cases per 1000 (5.6 more cases seen with paddock than shed reared calves)
Population attributable fraction where belong, what type of comparison, define, equation and sentence
Measures of total effect
RELATIVE COMPARISON
- the proportion of disease in the population that is due to exposure
- the probability that an individual selected at random from the population will develop disease as a result of exposure
PAF = RT - RE-/RT
- 31% of Johne’s disease cases in the population was attributable to being paddock reared (PAF = 0.31)
- if eliminate exposure decrease disease by PAF
What are the 3 main features of a cause
- Must precede the effect
- Can involve host or environmental factors
- Can be either
- Positive = the presence of exposure
- Negative = the absence of exposure (vaccination)
List and describe the 3 types of causes of disease
1) component cause
- pieces of the pie
2) sufficient causes
- the whole pie - a set of conditions without any one of which the disease would not have occurred (often several factors)
3) necessary cause
- the most important piece of the pie
- the condition that must be present for disease to occur
List some features of component causes and sufficient causes
- Component causes can act far apart in time
- A component cause can involve the presence of a causative exposure or lack of preventive exposure
- Blocking the action of any component cause prevents the completion of the sufficient cause and therefore prevents the occurrence of disease by that pathway
- Completion of a sufficient cause is synonymous with occurrence of disease
define a direct and indirect cause
Direct causes:
- No known intervening variable between the exposure factor and the disease
Indirect causes:
- Effect of exposure is mediated through one or more intervening variables
What are the 5 steps in determining whether something is causal
1) Risk ratios - is there an association between exposure and disease
2) Is there bias? - collection, misclassification - rule out
3) Confounders?? - rule out
4) Did we fluke it, is it by chance? - use statistics
5) Is the relationship between exposure and disease causal?? - JUDGMENT - subjective assessment so use criteria below
List the 8 Hill’s criteria
1) Strength of association
2) Consistency
3) Specificity
4) Temporality
5) Dose response
6) Plausibility and coherence
7) Experimental evidence
8) Analogy
Describe the following Hills criteria:
Strength of association, consistency and specificity
1) Strength of association
- Strong associations are more likely to be causal
- weak may be causal but harder to rule out bias and confounding, could be strong without causality
2) Consistency
- Has the cause and effect relationship been identifies by a number of different researchers
3) Specificity
- A single exposure should cause a single disease
- MANY exceptions
Describe the following Hill criteria:
Temporarily, Dose-response relationship and plausibility and coherence
Temporality
- Cause MUST precede effect - Can be difficult to establish
○ Long induction periods or latent phases of disease
- Prospective studies are less likely to confuse the issue of temporality
Dose-response relationship
- As the level of exposure is increased, the rate of disease also increases
- Be aware that there may be also non-linear effects
Plausibility and coherence
- Does the association make biological sense
○ More willing to accept if consistent with current knowledge
- Readier to accept arguments similar to others that we accept
Describe the following Hill criteria:
Experimental evidence and analogy
Experimental evidence
- Investigator-initiated interventions that modify exposure through prevention, treatment or removal should result in less disease
- Study designs in order of usefulness:
1. Randomised, controlled trails
2. Cohort studies - come opportunity to minimise bias
3. Case-control studies - subject to bias ○ Cross sectional studies not useful because they provide no direct evidence of the time sequence of events
Analogy
- Has a similar relationship been observed with another exposure and/or disease
Eg - BSE and scrapie - transmissible mink encephalopathy
What are the 3 main goals of epidemiological research
1) quantify the frequency of disease in a population
2) quantify the association between exposure(s) and presence (or absence) of disease
3) make informed judgements about causation
what are the two types of errors
- those that affect precision = random error
-> to get high precision all values need to be close to each other
those that affect accuracy = systematic error = deviation from the true value - due to bias
confidence intervals what does it do, how calculated
- provide an indication of the likely range of the population mean
- provide an indication of the precision of your measurement
- calculated using means and standard deviation of the sample means
what occurs as take more samples of animals in terms of sd
sd = standard deviation
gets smaller and therefore getting more certain about the data
what does systemic error do and is it bad
moves our measured value away from the true value
makes us more likely to make an incorrect inference about what we’ve observed - dangerous
What are the two main types of bias and types within
1) selection bias
1. surveillance
2. referral
3. non-response
4. length of stay
5. survival
2) Misclassification bias
1. recall
2. interviewer
3. prevarication
4. obsequiousness bias
Selection bias define
- Selecting individuals that are not a proper representation of the at risk population
- Caused by the procedures used to select units that are included in a study
- Occurs when selection procedures result in a different relationship between exposure and outcome amongst those that took part in the study compared with those who were eligible for inclusion but did not take part
Describe what is meant by 1. surveillance 2. referral 3. non-response 4. length of stay 5. survival bias
- looking in the wrong place for disease
- differential referral patterns are a source of bias in hospital-based case control studies (emergency centers)
- non-response or refusal to participate in a study can produce selection bias
- the longer you stay in hospital, the more likely you’ll be asked to take part in a trial
- long-term patients tend not to be representative of the general population of hospital patients - introduction of insulin has increased the lifespan of diabetic patients, producing an apparent increase in the prevalence of diabetes
List 3 ways in which you can avoid selection bias
- random selection of study participants from the study population
- ensure response rates are high amongst selected participants
- ensure that withdrawal rates are low among selected participants
Define misclassification bias and the two main types
- Misclassification bias arises from errors in the information that we record about our study participants once they’ve been selected for inclusion
1) non-differential = random - error in estimate of exposure for both control and non-control
2) differential = non-random - error in estimate of exposure for either control or non-control
- recall bias (more likely to remember exposure if diseased child)
- misclassification of one group over the other - BAD
Describe what is meant by:
- recall
- interviewer
- prevarication
- obsequiousness bias
MISCLASSIFICATION BIAS
1) interviewees better at recalling events when disease has occurred
2) potential problem when interviewer is privy to the hypothesis under investigation
3) subjects in a study may have ulterior motives for deliberately overestimating exposure to a hypothesised causal agent (e.g. compensation claims in workplace studies)
4) occurs when subjects systematically alter their responses in the direction they perceive to be desired by the investigator
List 4 ways to avoid misclassification bias
1) blind assessment of exposure and disease – i.e. assessing each without knowledge of the other
2) use of a rigorous and biologically valid method of assessing disease and exposure
3) use of complete and detailed sources of information (e.g. complete exposure histories)
4) use of objective measures where available (e.g. live weights, fleece weights, milk production records, pregnancy testing results, laboratory measurements)
define a confounder
- Confounding refers to the distortion of the true underlying relationship between an exposure and an outcome of interest, because of the influence of a third factor
Confounders ≡ ‘lurking’ variables
What are the 3 features of a confounder and therefore the 3 questions you must ask
1) It is causally associated with the outcome
2) It is causally or non-causally associated with the exposure
3) The confounding variable and the exposure variable are on two separate causal pathways to the outcome
what is effect modification, example and what used for
- Different from a confounder as interaction effect is based upon exposure to varying levels of the third variable
- Eg: effect of smoking on lung cancer is different dependent on the age of the individual
- Helps you define high risk groups
How to define the issue of a study, what approach used
PICO
P – What is the population of interest?
I – What is the intervention (e.g. exposure, treatment, diagnostic test) of interest?
C – What is the comparison/control group of interest? - what happens when don’t interfere?
O – What is the outcome of interest? - is the disease present? does it have a longer survival time?
What are the 3 broad types of studies and 2 types of time
Study type
1) descriptive
2) analytical
3) Experimental
Time
1) Retrospective - starting today and going back in time to look at reports, statistics etc.
2) Prospective - study going into the future starting today
Descriptive studies aim, what doesn’t it do
describing an outcome in a population/ individuals (PO questions)
•No comparison between study groups
- no conclusion about disease and exposure
- useful for generating hypotheses (NOT testing them)
Describe the 3 types of descriptive studies
1) Case reports: narrative of a ‘newsworthy’ clinical occurrence, e.g. unusual combination of clinical signs, experience with a novel treatment, or a sequence of events
- A case report
2) Case series: describes multiple cases and pattern, shows that it can happen repeatedly
3) Descriptive studies: reporting measures of health for a population
What are the advantages and disadvantages of descriptive studies
•Advantages - quick and cheap - fast turnaround -> quick dissemination of information - raising awareness - generating hypotheses •Disadvantages - no testing of hypotheses - limited evidence - low validity
Analytical studies aim, what do they do and list the 5 types
Aim: testing hypotheses •Typical epidemiological hypothesis: exposure A is associated with (or causes) outcome Z •Comparison between groups •PICO questions 1) Ecological 2) cross-sectional studies 3) cohort studies 4) case-control studies 5) Hybrid
what are some other terms for exposure and outcomes
Exposure
- synonyms: risk factor, putative cause, independent variable, predictor
Outcome
- synonyms: effect, end-point, dependent variable
Ecological studies what type of study, main features, advantages and disadvantages
Analytical study
The unit of analysis is a group (such as counties, states, farms, pens) rather than an individual
- ‘cross-level inference’ occurs when inferences are made at the individual (rather than group) level
Advantages
- quick and cheap
- data (often collected for other purposes) readily available
Disadvantages
- ecological fallacy: an association observed between variables at the aggregate level may not necessarily represent the association that exists at the individual level
Cross-sectional study what type of study, what occurs, what is the measure of health and association
Analytical study
- Sample of individuals from a population is taken at one point in time
measure of health = prevalence (not incidence as no new cases)
measure of association = odds ratio or RR of prevalence
What are the advantages and disadvantages of cross-sectional studies
Advantages
- quick and cheap
- feasible for large sample sizes
Disadvantages
- provide only a snapshot in time
•no information about incidence
•not appropriate for diseases of short duration - more likely to miss an occurrence of that disease
- difficult to investigate cause and effect relationships (establishing correct temporal sequence can be difficult)
○ What occurred first (cause) that lead to the effect
Cohort studies what type of study, what are the 4 steps in the study, what occurs, measure of health and association
Analytical study
1. Exposure status
- Are they exposed yes or no
2. Split into those groups
3. Follow the groups overtime
4. Are they becoming cases yes or no
- Following a group of individuals (cohort) over time, either prospective (starting with the animals then following) or retrospective (need to have really good records - still using the same steps)
Measure of health = incidence risk or rate
Measure of association = relative risk or odds ratio of outcome
List the advantages and disadvantages of cohort studies
Advantages
- following individuals over time -> incidence of disease
- temporal sequence of exposure (cause) and outcome (effect)
- suitable for rare exposures
- multiple outcomes
Disadvantages
- long follow-up period
○ expensive (cost, effort & time)
○ losses to follow-up (producing non response, migration and loss-to-follow-up biases)
- may require very large samples to ensure at the end of the study after losses there is still enough for statistical significance
- not suitable for rare diseases - numbers that will get isn’t feasible to run a study (time and money spent isn’t worth it)
- not suitable for diseases with long latency - may need to wait very long time
Case-control studies what type of study, what involved, measure of health and association and key issue
- Select a group of cases (outcome +) and non-cases (also called controls, outcome -) and compare frequency of exposure
- Looking back in time - retrospective looking at reports, can be prospective
Measure of health = None as cases and controls have been purposely selected, thus the number of cases does NOT represent the disease frequency in the population at risk
Measure of association = Odds Ratio of exposure
Key issue: cases and controls have to be similar in every way except for the exposure factors hypothesised to be associated with the disease of interest
what are the 3 main ways to prevent confoudnig factors in case-control studies
1) restricted sampling (one breed, age group, gender)
2) matching (selecting controls with similar characteristics - if using different breeds using the same breeds as controls)
3) control at analysis level (e.g. multivariable regression - test for confounding)
List advantages and disadvantages of case-controls studies
Advantages
- quick & relatively inexpensive
- suitable for studying multiple exposures
- suitable for studying rare diseases - selecting the cases of rare diseases, most involve controls that match
- suitable when randomisation is unethical (e.g. alcohol and pregnancy outcome)
Disadvantages
- provide no information about risk - cannot calculate relative risk of the population
- can be difficult to investigate cause and effect relationships
- dependent on clear case definitions & good records
- unsuitable for studying multiple outcomes
- vulnerable to confounding & bias
Hybrid study designs what type of study and give example
Analytical study
3) Panel study: series of cross-sectional studies with the same individuals (panel)
Repeating snap shots overtime with the same individuals
Experimental studies what are the main aims and features
- Aim: evaluation of interventions (e.g. specific risk factors, treatment options, diagnostic procedures)
- Investigator has direct control over study conditions
- Laboratory-based or field-based
- Intervention on level of individuals or communities (=community trial)
Randomised clinical trials what type of study
Experimental studies
1) use of a control group
2) randomisation - randomisation of participants to treatment and control groups
3) admission criteria
4) outcome ascertainment
5) Ethics
Why is use of a control group important in randomised clinical trials
the effects of treatment can be judged only in relation to what would happen in its absence (normally)
- conditions often improve without active intervention - why control so important
○ natural improvements in condition over time
○ placebo effect – positive effects because of an inert intervention - psychological effect
○ Hawthorne effect – positive effects because of observation - more likely in animals - Because animal is treated, getting more attention, treating differently (can make the animal better or just look better)
•use of a proper control group to serve as a baseline for comparison will help neutralise these effects
- Can have negative and positive control
why is randomisation important in randomised clinical trials
Groups being compared must not differ with respect to relevant characteristics (i.e. characteristics that determine the outcome) other than the exposure being studied
•Otherwise CONFOUNDING
•The most reliable way to encourage comparability is by assigning the treatment according to chance mechanisms (‘randomisation’)
- Any of the animals have an equal chance of being assigned to either group
what are the types of admission criteria in randomised clinical trials
- individual, place and time
- demographic (e.g. age, sex, race) - maybe just females or males
- prior conditions (e.g. having or lacking a particular disease or condition)
- risk factor restriction (non-smokers)
May need to restrict the study population before use randomisation
How to assure accurate measurement outcomes
objective measurements & blinding
Objective measurement
- Subjective (observation) measurement isn’t as good as objective (exact measurements)
Blinding
1. Of the study participant -> give the same thing to the treatment and control
○ Ideally blind the owner
2. The assessor or researcher -> they assess the outcome
○ Leads to double blinding
3. The statistician -> the person that is doing the maths, just tell them group A and group B
○ Triple blinding -> with this the results are most objective
what are the main features of Randomised controlled trails, advantages and disadvantages
Aim: test different types of treatment
Intervention: intervention is done by the researcher -> you are infecting the animals, randomly allocate intervention
Control group? Group given standard treatment or not treated
•Advantages
- excellent control over study conditions & confounders
- provide strong evidence
•Disadvantages
- may not be ethical or feasible
- expensive
- impractical if long periods of follow-up required
what is important in terms of ethics with randomised controlled trials
- You are putting the intervention and assigning that to the patients
- Need to consider whether this is ethical or not
- WHAT IS THE GAIN OF THIS EXPERIMENT???
It is unethical - to expose individuals to harm
- to withhold benefits (e.g. effective treatment) - better to give standard treatment to the control group and compare against that so still giving the control animals treatment
Animal research - scientifically valid & subject to ethical review
- 3Rs (replacement, reduction, refinement)
ALL COMES DOWN TO STUDY DESIGN
Systemic review what type of study, aim, key features
Experimental study
Aim: search and collate existing research to assess the evidence available to answer a clinical/research question
- Need to come out with good search strategy otherwise will miss some studies - bias already
- need to determine eligibility criteria
- may get language bias
what are the systemic review advantages and disadvantages
Advantages
- combine results of several studies
- objective
- reproducible
- larger sample sizes
- greater external validity -> results together are more applicable to more populations and groups
Disadvantages
- pooling of results, sample sizes - as may compare two things that shouldn’t be compared
- misleading due to low quality or bias present in primary studies - critical analysis of the studies chosen to avoid this
List the study designs you would choose
1) to determine if there is a health problem on the farm?
2) to investigate risk factors for clinical mastitis in dairy farms
3) to describe a rare disease
4) to evaluate a new surgical method for left displaced abomasum in dairy cattle
5) to investigate risk factors for pelvic fractures in race horses
1) cross-sectional study
2) cohort study
3) case-report or series
4) experimental randomised trial
5) case control as rare
What are the 4 steps involved in critical appraisal
- Describe the evidence
- Assess the internal validity of the study
- Assess the external validity of the study
- Compare the results of the study with other available evidence
What are the 4 steps involved in critical appraisal
- Describe the evidence
- Assess the internal validity of the study
- Assess the external validity of the study
- Compare the results of the study with other available evidence
what are the two aspects of internal validity of a study and features within
1) non-causal explanations •bias - selection and missclassification •confounding •chance - is it a fluke, statistically significant 2) causal explanations •temporal relationship •strength of relationship •dose-response •consistency •specificity (Hills criteria)
what occurs if severe bias or confounding is present in a study
- if there’s severe bias, no analytical manipulation of the data will overcome the problem
- if there’s confounding, then appropriate analysis will (in most cases) overcome the problem
- assessment of chance variation should be made after considering issues of bias and confounding
external validity of a study what are the 3 aspects that should be considered
: can the results be applied to populations other than that which was studied?
- if the internal validity of a study is poor, the answer is no
1. Can the results be applied to the eligible population? - losses due to non-participation have to be considered carefully
2. Can the results be applied to the source population? - not whether the subjects studied are ‘typical’, but whether the association between outcome and exposure given by the study participants is likely to apply to other groups
3. Can the results be applied to other relevant populations?
what are the two main problems leading to the study population being different from the source population
1) Ineligible population - need to do a good job in selecting eligibility
2) Decline to participate - different group of people that say yes and no
○ Participation rates are important especially in the veterinary papers
Comparison with other evidence for studies what involved
- need to consider a hierarchy of evidence
1. Are the results consistent with other evidence?
2. Are the results plausible biologically?
3. Is there coherency with the distribution of the exposure and the outcome?
Remaining 3 Hills criteria
What is the difference between cross-sectional and cohort compared to case-control studies in terms of odds
Case control start with outcome so calculate odds of exposure in diseased (=a/c) compared to the odds of exposure in non-diseased individuals (=b/d)
Cohort and cross-sectional start with exposure and found out outcome so calculate odds of disease in exposed individuals (=a/b) to odds of disease in the unexposed ones (=c/d).
HOWEVER (OR of exposure and OR of disease respectively) works out to be the same for both study designs (OR= ad/bc).
How to identify a cross-sectional study
1) Cross-sectional studies are designed to answer PICO questions, meaning that they aim to examine association between exposure(s) and outcome.
2) Abstracts of cross-sectional studies often only state how exposure and outcome were measured, e.g. a questionnaire was applied (asking questions about exposure and outcome) or a farm was visited once and data (on exposure and outcome) was recorded.
AT THE SAME TIME
when a questions says what proportion of something is due to something what does it want and equation
The population attributable fraction
Equation depends on population the question is asking about
what proportion of disease in positive exposed then
The attributable fraction = (RE+ - RE-) ÷ RE+
If what proportion of disease in total population then The population attributable fraction = (Rtotal - RE-) ÷ Rtotal
What are the two main important spartial factors that influence patterns of disease in a population
1)Environmental factors include aspects of climate (temperature, humidity, rainfall)
2) aspects of management
(e.g. management of animals in a certain area of a country may result in high rates of disease
that may not be seen in other areas)
herd of 200 cattle and over a 12-month period we identify 150 animals with disease. What is the odds of disease
150:50 or 3 to 1.
That is, there were three times as many
diseased animals in this herd as there were undiseased animals.
What are the two types of prevalence and how to calculate
1) point prevalence equals
the proportion of a population in a diseased state at a single point in time - cross-sectional studies
2) period prevalence equals the proportion of a population with a given disease or condition over a specific period of time.
When calculating period prevalence the number of cases equals the number of individuals which have the disease at the start of the period plus the number of new cases that occur during the
follow-up period.
What is assumed with open populations and determining incidence risk
If an individual can only experience one episode of disease we include diseased individuals with the group that leave (i.e. once they’ve become a case they are removed from the population at risk).
Assume assume that once an animal becomes a case it is no longer susceptible to disease
If the population is closed or open what are the key things to remember when calculating population at risk
1) If the population is closed the population at risk equals the number of disease free individuals
present at the start of the follow-up period.
2) If the population is open the population at risk should be adjusted to account for those that
enter and leave the population throughout the follow-up period.
What are the 2 main differences between prevalence and incidence
Prevalence
Num - Cases present at period start + new cases during follow-up period
Dem - all individuals examined
Incidence
Num - New cases during the follow-up period
Dem - All susceptible individuals present at the start of the study
What describing odds what is the difference with cohort and case-control studies
In a cohort study we talk about the odds of disease being x times greater (or less) in the exposed, compared with the unexposed.
In a case-control study we talk about the odds of exposure being x times greater (or less) in cases, compared with controls
What two things determine the magnitude of PAF for a given exposure
1) the prevalence of exposure
(2) the strength of the association between the exposure and the outcome
Just because a cause is necessary does that mean it is sufficient
NO
infection with Mycobacterium tuberculosis is a necessary cause for tuberculosis it is not a sufficient cause since many animals may harbor small foci of tuberculosis without developing tuberculosis
what measurement is used for strength of association and general guidelines for strong
Strong associations are usually considered to be risk ratios in excess of 4 or 5. The term ‘small’
is used in epidemiology to describe risk ratios from observation studies that are less than 2.0, since
it is possible that such associations may be due to bias and/or confounding
What are the 3 ways to reduce random error
- Increasing the size of the sample taken. Using the central limit theorem it can be shown that a increasing the sample size by a factor of four will half the width of the confidence interval.
2) Modifying the sample selection procedure to ensure that only the target group is sampled. For example, you may be interested in the performance of only one particular breed of dairy cow.
3) Using an appropriate scale of measurement.
How does a retrospective cohort study differ from a case-control study?
In a case-control study one identifies cases and then selects a set of appropriate controls. An entire cohort is not used.
In a retrospective cohort study one uses the entire cohort — all cases and non-cases within the identified group.
What is a nested case-control study and advantage and disadvantages
iHYBRID STUDY - similar to a cohort study with the key difference that a sample of non-cases are selected for analysis (rather than the entire cohort).
AD - useful when it is either too costly or not feasible
to perform additional analyses on an entire cohort
DIS - if don’t use all participants may get variation and no true representation of source population
What are the 4 main populations in a study and define
- The study population: individuals who actually took part in the study.
- The eligible population: individuals who met the criteria to be included in the study.
- The source population: the population from which eligible study subjects were drawn.
- The external population: individuals not part of the source population (e.g. those in another region or country) but who, based on the results of the study, you want to make generalisations about.
