types of study Flashcards

Question

how is an ecological study done?

Answer 1

* Compares group averages – health and risk factors * Background risk will practically always differ between groups (groups are mostly not made up randomly) * Limit inferences from ecological studies * Explore differences between exposed and non-exposed But * Take advantage of natural experiments * Data that are already available

Answer 2

An attempt to infer from the ecological level to the individual level is often called an 'ecological fallacy'

Answer 3

ecological inferences about effects at the group-level> they don’t enable us to make inferences of individual risk

Answer 4

data collected routinely, in a standardised and consistent way, for administrative purposes rather than targeted in a specific study

Answer 5

Advantages - Often covers large populations, even whole countries - Readily available + Cheap as already collected Disadvantages - Often not up-to-date given reporting delay and processing time - Can be incomplete (except for census) - Variable of interest may not be collected - Can be influenced by political pressure or financial constraints

Answer 6

- To generate Hypotheses - To assess a population and describe its baseline characteristics - To estimate disease prevalence or incidence of events -> sample size calculations when planning detailed studies

Answer 7

- uses anonymised personal identifiers - to link o mortality o hospital discharge o prescribing (Tayside) o laboratory tests (Tayside) - to examine o predictors of mortality o quality of care o adverse drug reactions

Answer 8

Measures ‘todays’ risk factors and then follows up Need to keep good records of both groups into the future More expensive and time consuming but there is a defined cohort with detailed records

Answer 9

Measures the outcomes of interest ‘today’ Need to obtain detailed enough records of both groups looking at the past Get faster answers, cheaper as follow up doesn’t have to be done, need to investigate quality of the past records

Answer 10

atypical study group losses to follow-up

Answer 11

1. Infrequent/unusual exposure e.g. radiation 2. Multiple outcomes related to infrequent exposure 3. Sometimes for a rare outcome - follow-up power 4. Temporal sequence needs to be established 5. Interested in risk over time (e.g. change in risk with more years of smoking)

Answer 12

1. Decide the study question 2. Define target population 3. Select study population 4. Measure exposure 5. Determine appropriate method to determine outcome 6. Consider potential confounders and how you can measure them 7. Logistics of follow-up – determine how much is needed 8. Determine appropriate statistical analyses for association testing 9. Interpret results

Answer 13

1. Questionnaires 2. Laboratory tests/physical measurements 3. Medical records 4. Occupational records 5. Civic/governmental records

Answer 14

Must ensure outcomes are measured in the same fashion for exposed and non-exposed. Tendency to look more closely in exposed group differential bias 1. Clinical assessment 2. Questionnaires 3. Medical records 4. Government/civic records

Answer 15

participants are identical in all aspects but exposure

Answer 16

related factors to the outcome and exposures – not in the causal pathway. You can control for confounders, accurate analysis need to collect data on all confounders and adjust the analyses accordingly

Answer 17

- Compare to a comparable population (age, gender, socio-economical class) - Use general population data on incidence rates

Answer 18

Step 1: make a contingency (or 2x2) table Step 2: what kind of question do you want to answer? * what is the risk of outcome for those exposed? * What is the comparative risk of outcome for those who are exposed compared to those who are unexposed? * Can we calculate a “rate”? This requires a time component… unique advantage of cohort studies is follow-up…

Answer 19

risk ratio (RR) assess the strength of the association – compares incidence of disease between exposed and unexposed

Answer 20

measures clinical and public health importance of the causal relationship

Answer 21

* RR >1 – exposure is harmful * RR <1 – exposure is protective * RR = 1 – exposure does nothing (risk ratio = 15.6 then risk of disease is 15.6 times higher than those without exposure)

Answer 22

Cases – have disease of interest Controls – do not have the disease Where cohort studies measure disease in exposed vs unexposed, case control studies SELECT (diseased) and (undiseased) controls. Starts with disease then finds exposures

Answer 23

1. decide sample size 2. case selection 3. control selection 4. collect exposure 5. data analysis - odds ratio 6. deal with the validity of threats

Answer 24

o Selected cases should be truly representative of all cases in source population, which you sample your control group from o Clinic based sampling is vulnerable to selection bias, incidence rate calculation is not possible and the generalizability is uncertain compared to population sampling o Cases must be sampled independent of exposure, if association known/suspected can be introduces. Intensified monitoring in exposed vs unexposed (higher likelihood of diagnosis in exposed then, exposure avoidance if high risk of disease = lower likelihood of exposure in diseased)

Answer 25

relation between exposure and disease is different for those who participate and those who are theoretically eligible but do not – procedures to select subjects or factors which might influence

Answer 26

Occurs when errors in the measurement of characteristics and consequences of the errors are different for exposed vs unexposed/cases vs controls

Answer 27

a higher likelihood of diagnosis in the exposed a lower likelihood of exposure in those with risk factors for the disease (e.g. patients who had an episode of the outcome of interest before = prevalent cases)

Answer 28

true case wrongly labelled control true control wrongly labelled case eg if knowledge of exposure status influences diagnosis - Impact if random - usually weakens observed effect if associated with exposure – unpredictable - Minimise – ensure accurate objective diagnosis

Answer 29

Knowledge of case/control status may influence data collection  Impact – identify more (spurious) risk factors in cases  Minimise by Use of standardised objective instruments Blind researchers to case/control status

Answer 30

Cases and controls recall prior exposures differently  Minimising Recall Bias Minimise period of recall (if possible) Measure exposure data objectively - Medical notes or Third-party verification of exposure information

Answer 31

If exposure is rapidly fatal what happens to number in cell a* (patient dies before he/she becomes known as case) Will detect factors that increase survival among the diseased as risk factors for the disease

Answer 32

stratification  Advantage – effectively controls confounding  Disadvantages – reduces power and makes presentation and interpretation complex Matching cases and controls  Advantage – effectively controls confounding  Disadvantage – matching on several factors makes it hard to find controls and cannot explore association of disease and matched variable Multivariate methods (logistic regression)  Advantage – can cope with, several factors at once and continuous and categorical data simple if unmatched conditional logistic regression for matched  Disadvantage/Caution - May hide effect modification by treating effect modifiers as a nuisance

Answer 33

a factor which modifies the effect of exposure to a risk factor ie exposure to the risk factor has different effects at different levels of the effect modifier

Answer 34

Risk of disease given exposure (like the RR in cohort studies) in most circumstances

Answer 35

genes are sections of DNA which determine our characteristics they work and can either cause disease mendelian or with poly genetic be a contributing factor to disease

Answer 36

disease mendelian and polygenic can increase and decrease risk of disease certain characteristics more at risk - more at need for prevention in some groups

Answer 37

study of families - Familial Aggregation Analysis – are relatives of a person with the disease more likely to have the disease than the general population? - Twin studies – Quantifies the relative contributions of genetic and environmental factors to a disease - Adoption Studies - An alternative method for establishing the relative contribution of genetic and environmental factors to a disease – Compares disease concordance with biological parents to concordance with adoptive parents - Segregation Analysis - Analyses the mode of inheritance of a disease and how many genes are involved

Answer 38

can focus the advice on people with a higher genetic likelihood for disease screening can be carried out genetic testing of family members were prevention is possible

Answer 39

is it ethical for family members et. issues like abortion and having kids

Answer 40

total number of deaths/ total population for specific time period

Answer 41

- depends on the age/sex structure of the populations - country with a higher proportion of old people will have higher number of deaths and higher crude mortality rate

Answer 42

often used to control for confounding effects of age so that the rates of disease or mortality can be compared in populations with different age structures

Answer 43

new death rate - if age specific rates had occurred in the standard population - obtain an age standardised rate which adjusts for the effect of age

Answer 44

o the age-specific rates for all populations under study (or the data to calculate them) o an appropriate standard population with a known age distribution

Answer 45

number of deaths expected if both populations had the same (standard) age-specific death rates, but kept their real age structure

Answer 46

o Age-specific mortality rates for a standard population o The age structure of the study populations o The total number of deaths in the study populations

Answer 47

a measure expressed as either a ratio or percentage, to quantify an increase or decrease in mortality in a study cohort compared to the general population

Answer 48

= O / E * 100 Compares observed (O) with expected (E) deaths Input needed - Number of persons in each age group in the population being studied - Age specific death rates of the general population by the same age groups - Observed deaths in the study population

Answer 49

Advantages - Less bias - Better when comparing 2+ groups with different age distributions (age-specific rates of study populations applied to same standard population) Disadvantage - needs age specific rates for study populations and these are not always available or reliable (if based on few cases in some age groups) - In small, subpopulations with few cases Indirect method is preferred over the direct method

Answer 50

Advantages - only need Total observed cases/deaths in study population and its age structure Disadvantage - SMR depends on age distribution of study population - (since age-specific rates of standard population applied to each study population by age group) - SMRs can only be compared between populations with similar age structure

Answer 51

* Developing and developed countries * One country at two time periods * Occupational groups * Social class

Answer 52

1.The confounding factor must be associated with both the risk factor of interest (exposure) and the outcome. 2. The confounding factor must be distributed unequally among the groups being compared (E+ and E-) or (O+ and O-). 3.A confounder cannot be an intermediary step in the causal pathway from the exposure of interest to the outcome of interest.

Answer 53

1. Must collect information on all known potential confounding factors 2. Explore for confounding in the analysis 3. Practically, difficult to know which are the important confounders so you check magnitude 4. Once identified, adjust regression models for confounders (multiple or multivariable regression)

Answer 54

- Confounding will give you a flawed answer - May account for all or part of an apparent association - May cause an overestimate of true association (positive confounding) or an underestimate of the association (negative confounding)

Answer 55

- need to determine as whether the finding is because of chance or what is being measured - Null hypothesis (H0) v. alternate hypothesis

Answer 56

1. Propose a hypothesis, and by extension propose the null hypothesis i.e. no effect 2. statistical test and calculate P value - 95% CI: If p value < 0.05 à conclude chance is unlikely. Therefore, the effect is real or true. - If p>0.05 cannot exclude chance ie cannot conclude there is real effect The meaning of p<0.05 If p=0.05 would get result (as extreme) 1 time in 20 If p=0.01 would get result (as extreme) 1 time in 100

Answer 57

* Most of the time (95%) the confidence interval will contain the real value We know i) observed treatment difference ii) our result is affected by chance We need to know i) where the true value might lie

Answer 58

* For difference in mean treatment effect o if zero within confidence interval – NOT significant * For ratio measures eg relative risk o no difference if ratio = 1

Answer 59

- relative risk - hazard ratio - odds ratio

Answer 60

attributable risk population attributable risk

Answer 61

difference between the incidence in the exposed and that in the unexposed incidence in exposed – incidence in unexposed AR= ie-iu

Answer 62

cohort study - RR-1/RR x100 * There is no way to calculate AR in a case-control study However if - Exposure is not very common - Risk is not very high Then – odds ratio approximates relative risk

Answer 63

incidence of disease in a population attributable to the risk factor

Answer 64

* absolute difference between risk in the total population (It) and unexposed population (Iu) PAR = It – Iu PAR = AR x Pe (Pe – prevalence of exposure in whole population)

Answer 65

* excess risk of disease in total population attributable to exposure * reduction in risk achieved if population entirely unexposed * helps determining exposures relevant to public health in community

Answer 66

* proportion of cases in the population attributable to the exposure * PAR expressed as a percentage of total risk in population * Proportion of the disease in the population that could be eliminated if exposure were eliminated

Answer 67

Problem – getting the data * Incidence in the total population - From routine data * Incidence among the exposed and non-exposed - Use data from cohort studies - Assume it applies to the whole population

Answer 68

implies that there is a true mechanism that leads from exposure to disease

Answer 69

* A factor which increases the frequency of a disease (outcome/event)

Answer 70

* No single causes of disease * Cause of disease – a constellation of components that act together * 2 types of cause – necessary and sufficient

Answer 71

- An exposure which is necessary for disease to occur - A necessary cause must always precede the disease – HIV and AIDS - Necessary cause may not act alone – not everyone with HIV gets AIDS

Answer 72

- Set of conditions is a sufficient cause when it always produces the outcome

Answer 73

1. Strength of the association 2. Consistency - replication 3. Specificity of the association 4. Temporal sequence 5. Biological gradient 6. Coherence 7. Experiment 8. Theoretical basis 9. Analogy

Answer 74

* A given sufficient cause requires the joint action of many component factors (component causes) * Sufficient causes can vary between individuals * component causes can act far apart in time. * a strong factor is a component of many causal pies * blocking the action of any component cause prevents the disease

Answer 75

randomised controlled trials - randomisation enables fair comparison - need a fair outcome assessment

Answer 76

PICO population intervention comparator outcome

Answer 77

unequal at baseline lack of blinding loss to follow-up poor outcome measurement

Answer 78

useful if other risk factors have a strong influence on the outcome

Answer 79

refers to any procedure used with random assignment to achieve balance between study groups in size or baseline characteristics

Answer 80

* Each patient gets both treatments, with half receiving A first and half receiving B first * Patient is own control o reduced variance o much smaller sample size * Requirements o no carryover A to B or B to A o no change in disease severity

Answer 81

* when people are organised in natural groups * cluster are randomised not individuals complications * Need to randomise lots of clusters – If few clusters could get unequal distribution after randomisation i.e. not the same at baseline  * Need increased sample size (of individuals) – subjects within a cluster, more similar than subjects in other clusters, Intra-class correlation coefficient – ideal is lots of small clusters * Allocation concealment more difficult * Analysis more complicated – need to take account of cluster design

Answer 82

* Cannot deliver to all clinics at same time o Randomise clinics into a sequence for treatment o All clinics receive intervention but not at same time * Possible uses o New treatment – clinic by clinic o Provision of piped water, free school meals or prisoner vocational training (always place by place)

Answer 83

* Time of crossover is randomized ◦ crossover is unidirectional * Need to be able to measure outcome on each unit at each time step at same time * Same disadvantages as cluster randomisation ◦ best with lots of clusters/ steps ◦ analysis complex * Units act as their own control ◦ helps reduce no. units needed (same as cross-over design)

Answer 84

* Intervention and control groups will be similar in all respects except the intervention minimising selection bias and confounding * If the participants are "blind" to the treatment allocation, reporting bias is minimised; if the investigators are "blind" to the allocation, observer bias is minimised * RCTs carry less risk of bias and confounding than other study designs and so can provide powerful evidence of a causal relationship between the intervention and the outcome * Intervention studies are similar to cohort studies in that: o multiple outcomes can be examined o the incidence rate of the outcome can be measured

Answer 85

* Expensive to conduct: they may require a large study team, perhaps at several sites, and may require a long follow-up period. * In some situations, intervention studies are impossible to conduct for ethical or logistical reasons. * Recruitment is difficult and time-consuming * Trials take years to do

Answer 86

* Compares outcomes for all randomised individuals – even if they stop taking treatment or drop out of study. * Assesses the overall effect of assigning a subject to receive a particular intervention. * Analysis is the most important and "safest" analysis as intervention and control groups compared as originally randomised. * More likely to underestimate treatment effect * Often use a modified ITT analysis – all randomised participants with some follow up data

Answer 87

* Opposite of ITT * Includes only those who finished the trial and took the drugs / underwent the intervention * Maximises likelihood of showing a difference between groups * May minimise harms * May introduce bias due to selective dropout (more on dropout later)