Epidemiology Flashcards
what are the 2 types of epidemiology? and define them
Descriptive epidemiology: providing measures of frequency
Analytic epidemiology: testing hypotheses and associations
What is confounding? and what does it lead to?
effect of an extraneous variable
that wholly or partially accounts for the apparent effect of the study exposure or that masks an underlying true association
- Can lead to biased findings
- Can produce misleading results
what are the ways of identifying confounding in am epidemiological study?
Knowledge of subject matter
See whether the variable follows the 3 conditions for confounding
Stratification
Compare crude and adjusted estimates
N.B- You only need one method to identify confounding
Methods of identifying confounding:
How do you expand your knowledge of subject matter
- Explore literature
- Knowledge of similar biological pathways can be applied
- Not always possible, however, especially when investigating novel associations
Methods of identifying confounding:
What are the 3 conditions for confounding in a variable?
Check whether the variable is:
- Associated with the exposure in the source population
- Associated with outcome in the absence of the exposure
- Not a consequence of the exposure (in the causal pathway)
Methods of identifying confounding:
How do you use stratification and describe what it entails
- Stratify data by the variable of interest
- Compare stratum specific estimate with the estimate from the data analysis
- When a pooled estimate is significantly different (10%) from stratum specific estimates it is reasonable to think there is confounding
Methods of identifying confounding
How do you compare crude and adjusted estimates?
- Create a regression model adjusted for the variable
- If adjusted odds ratio differs from the crude odds ratio by 50% or more this may indicate confounding
- Not the optimal method though as adjusting for stuff may introduce confounding.
What is effect modification?
- Exists when the strength of the association varies over different levels of a third variable
- After controlling for confounding there is still a variable which affects the exposure or outcome
- This is a natural phenomenon
What are the stat tests for effect modification (to confirm that stratum specific estimates are truly different between them)
- Breslow-day test
- Q test
- Interaction terms in regression models- very frequently used. Interactions is synonymous to effect modification
what can you do about effect modification?
Do not try to control it; it is not a problem as it occurs in nature.
Instead take it into account and present stratified results
This effect can occur when you further stratify groups of exposure
in effect modification what is Synergism and Antagonism?
- Synergism = effect modifier potentiates the effect of the exposure
- Antagonism = effect modifier diminishes the effect of the exposure
what is the difference between confounding and effect modifier
Addressing a confounded relationship by addressing the exposure exclusively is very unlikely to yield a gain.
Addressing an exposure where effect modification is apparent may be useful. Hence interventions could be targeted ti a more homogenous pool of participants.
Effect modification affects exposure or outcome but not both whereas confounding could independently affect both exposure and outcome
what is a crude model of analysis
Univariate
It simply looks at the impact of the exposure on the outcome with no consideration of anything else
what are the features of multivariate analysis
Uses adjusted models- multiple exposures have been included.
The inference is that the outputs of these analyses mean that holding all other adjusted variables equal, X is the association between exposure and outcome.
e.g adjusted odds ration or adjusted hazard ratio.
it can help us to find confounding
what are koch’s postulates for infering causation
- Microorganism must be found in abundance only in diseased
- Microorganism must be isolated from diseased and grown in pure culture
- Cultured organism should cause disease when introduced to healthy organism
- Ethical problems here
- Must be reisolated from experimental host and identified as same causative agent
We do not use koch’s postulates for inferring causation, hence what criteria do we use to infer causation from both observational and interventional methods?
LIST THEM
Bradford-Hill Criteria
- Strength
- Consistency
- Specificity
- Temporality
- Biological gradient
- Plausibilty
- Coherence
- Experiment
- Analogy
Bradford hill criteria- EXPLAIN the following terms and give any relevant details:
- Strength
- Consistency
- Specificity
- Temporality
Strength
- Stronger association increases the confidence that an exposure causes an outcome
Consistency
- Consistent findings across settings tend to rule out errors or fallacies that might befall one or two studies
- Meta-analysis is a summation of this approach
Specificity
- Describes an association between specific causes and specific effects
- One of the most criticised criteria
- Lack of specificity does not necessarily invalidate a causal relationship
- Difficult when the disease is multifactorial
Temporality
- Insufficient for exposure A and Outcome B to co exist; A must precede B
- Not useful for cross-sectional studies
- Longitudinal studies are more useful
Bradford Hill criteria:
Explain the following terms and give relevant details:
- Biological gradient
- Plausibility
- Coherence
- Experiment
- Analogy
Biological gradient
- Dose-response effect (in the ‘right direction’) is a compelling argument for causality. e.g smoking and cancer
Plausibility
- This more intuitive
- Relationship should be biologically plausible where the science is understood
- However, where there is deficient understanding, assessing whether a relationship is plausible or not may not be possible
Coherence
- Association should be consistent with the existing theory and knowledge
- Can be an issue when challenging current beliefs or questioning the status quo
Experiment
- Evidence from experimentation should be supportive of the proposed link
- However scientifically desirable, experimentation is often not ethical when dealing with public health issues
Analogy
- Drawing upon analogous findings, we may make inference on the relationship
- Important in understanding emergent diseases and new associations
Define correlation
Correlation is a statistical term describing a linear relationship between two variables
Validity and bias help us to determine whether a results from a study is relevant or trustworthy
What are the two types of validity and explain them
Internal validity
- The extent to which findings accurately describe the relationship between exposure and outcome in the context of the study . i.e. if an association truly exists in the study
External validity
- The extent to which these inferences can be applied to individuals outside the study population
- Internal validity is a prerequisite for this
- Sometimes referred to as generalisability
what is bias
Inference is valid when there is no bias
Bias is any trend in the collection, analysis, interpretation, publication or review of data that can lead to conclusions that are systematically different from the truth
What are 2 types of errors and can it lead to
Random and systematic error
- Random error can be overcome with a large enough sample size
If there is a systematic error, this leads to incorrect results regardless of sample size
Systematic error can introduce bias into a study
This reduces its validity
what are the types of bias
- Selection sias
- Information bias
- Confounding
what is selection bias and what studies are particularly susceptible to this type of bias
An individual’s chance of being included in a study sample may be related to both exposure and outcome
This leads to a biased estimate of the association between exposure and outcome.
Case-control studies are more susceptible to this
Describe and explain the variations/examples of selection bias
Berkson’s bias
- When there is a hospital-based case-control study
- Controls are selected among the hospitals patients
Healthy worker effect
- Active workers are more likely to be healthy than those who are retired
Non-response bias
- People who do not respond are systematically different to the people who do respond
what are the ways of mitigating selection bias
- Choose controls representative of target population
- Keep non-response to a minimum
- Compare respondents with non-respondents and explore any systematic differences between them
what is information bias ?
Misclassification of exposure or the disease status or both (placing people in wrong groups)
Usually to poorly defined variables or due to flaws in data collection.
what are the types of information bias and how do you mitigate them?
Interviewer bias
- Common flaw occurs when interviewers ask about exposure status
- May be more thorough with those how have the outcome desired
- Leads to misclassification of exposure status and biased odds ratio
Recall bias
- Specifically refers to the differentially inaccurate recall of past exposure between cases and controls
- Patients with outcome may be more likely to recall exposure
- We often forget past exposure, but try harder remember if we have disease
If all participants cant remember exposure there is no recall bias
How do we mitigate recall and interviewer bias?
Interviewer
- if the interviewer does not know the disease status
- If interview process is standardised so interviewers follow strict protocol
Recall:
- Prevent by using objective ways to assess exposure
- Eg. medical records or biomarkers
what are the two different types of misclassification and explain how it affects the odd ratios
Non-differential misclassification. when there are errors in determining the outcome but they happen equally among exposed and non-exposed groups.
Odds ratios ALWAYS biased TOWARDS the null.
Differential misclassification- when errors in determining an individual’s exposure status occur unevenly amongst cases and controls.
odd ratios may be bias towards or away from the null
what is the Lake Wobegon effect and Hawthorne effect?
Lake Wobegon effect
- Illusory superiority
- If you ask a class of students how good their driving is, most of them will believe themselves to be better than average
Hawthorne effect
- Consequence of participants realising they are being observed and therefore acting differently
what are the types of disease prevention? explain them
Primary- prevent disease by controlling exposure to risk factors
Secondary- apply the available measures to detect early departure from health and giving treatment and interventions
Tertiary- introduce treatment to reduce long term impairments and disabilities
Draw out the Demographic transition model with all the relevant stages
what are the stages of EPIDEMIOLOGIC transitions?
Draw it out
Describe the features of Stage 1 epidemiological transitions
Pestilence and famine
There is constraints on food supply
High birth rate and mortality
Life expectancy low at birth
what are the features of stage 2 of epidemiological transitions
Receding pandemics
A huge natural increase in population
High BR and reducing mortality
Agricultural development improves nutrition
Vaccination emerges at the end of this
what are the features of stage 3 of epidemiological transitions
Degenerative and man-made diseases
lifestyle factors and non-communicable disease dominate
Tech reduces need for physical labour
Addiction, violence and other issues emerge
What is Stage 4 of epidemiologic transitions
Give features of this
Delayed degenerative disease and emerging infections.
Health tech defers morbidity
Emerging zoonotic disease presents new threats
Inequalities within and between countries come to the lore
outline the hierarchy of evidence
what is DALY
(DALY) is a measure of disease burden that combines years of life lost from ill-health, disability or premature death.
People with low socio-economic status suffer the most as they don’t have resources to mitigate it
what are the 4 measures of frequency?
- Odds
- Prevalence
- Cumulative incidence
- Incidence rate
what is odds and how do you calculate it
Definition: a ratio of a probability of an event (P) to the probability of it’s complement (1-P)
Equation: (number of people with the disease)/ number of people without the disease.
what is the equation for prevalence and what are the features of it
Timepoint is very important in this
it reflects both the occurrence and duration of the disease
Can be used to monitor trends of disease overtime and hence to allocate health diseases
Not suitable for diseases of a short duration
what is cumulative incidence, give the formula and what factor is very important in this
used to measure how many new cases of the disease are there over a time period.
Time period is very important
it is a proportion; 0= no new cases in that time period and 1 suggest all individuals developed it during the time period
What is the other name for cumulative incidence and what must you do to make sure your calculation is correct
What are the limitations of using this:
Can also be called risk or incidence proportion
You have to follow up ALL participants
NO new participants should enter.
Limitations:
- People can drop out of study or die
- New participants may enter
what is the incidence rate? give formula
Number of new cases per unit of person time.
Ranges from 0 to infinity
Person time- a measure of the time spent in the study by participants. it starts when they enter the study until they get the disease, die or leave the study
what are the two types of standardisation? explain them
Direct standardisation- gives comparable incidence and allows us to adjust for differences in population
Indirect standardisation- this gives a ratio out of 100
You can standardise for age, sex, etc
For hospital deaths, what is the name of indirect standardisation used when comparing data
Standardise mortality ratio when it is adjusted for types of procedures
This is observed death/expected deaths using indirect standardisation
why are the reasons for a high death rate in a hospital? what data stats can you use to help mitigate these differences
Reasons are:
- Unwarranted variation
- Explained variation like higher amounts of high risks procedures
- Statistical artefacts- recording deaths
Indirect standardisation helps to account for these variations especially the standardised mortality ratio