Exam Revision Flashcards
What is the appropriate measure to use in cohort studies?
Risk ratio
How do you calculate risk ratio?
Risk in exposed = a/a+b
Risk in unexposed = c/c+d
RR = Risk in exposed/risk in unexposed
How do you interpret risk ratios?
RR>1 - harmful effect
RR = 1 - no effect
RR<1 - positive effect
What would be required to fully interpret risk ratio?
95% confidence interval is required to determine if the result is statistically significant
What are the advantages of cohort studies?
- Temporality (exposure precedes the outcome)
- No recall bias involved
- Can study multiple outcomes which are associated with a rare exposure
- It is possible to estimate all measures of incidence (rate, risk) and effect (risk difference, risk ratio)
What are the disadvantages of cohort studies?
- Requires a large investment of time, human and financial resources
- Requires a large sample size
- Reproducibility is hard, unless someone has a similar study
- Loss to follow up - bias introduced is difficult to control for (selection bias)
- Inefficient for rare diseases
- Uncontrolled confounding (unmeasured day-to-day exposures)
- Information bias due to misclassification of exposure or outcome
What is an example of a prospective cohort study?
Framingham study (cardiovascular cohort study)
What are the advantages and disadvantages of a prospective cohort study?
Advantages
- They can be followed up to any point in the future
Disadvantages
- Time consuming
- Expensive
- Defined cohort so requires detailed exposure records
- Records must be maintained well for both exposure groups
What is an example of a retrospective cohort study?
Asbestos factory and mesothelioma
What are advantages and disadvantages of retrospective cohort studies?
Advantages
- Can identify a cohort/determine exposure and continue to follow up
- Faster answers
- Dont have to employ people for regular follow up so is therefore cheaper
Disadvantages
- Since exposure is determined after the fact, quality of records must be checked
What is an example of a case-control study?
Probability of lung cancer in smokers
What is the appropriate measure for case-control studies?
Odds ratio
How do you calculate odd ratio?
a x d/b x c
What are the advantages of a case-control study?
- Allows for the study of rare diseases (case control useful for identifying current cases and looking at historical data as opposed to doing a large cohort study which would be expensive)
- Design allows to look at multiple risk factors at once
- Can be helpful when disease outbreaks occur, and potential links and exposures need to be identified
What are the disadvantages of a case-control study?
- Selection bias which is when the selection of individuals, groups or data is in a way that proper randomisation has not occurred
- Information bias which occurs when there are errors in the measurement of characteristics and the consequences of the errors are different for different subjects
- Recall bias
- Observer bias in data collection but can be solved by ‘blinding’
- Misclassification bias
- Confounding
What are advantages of randomised controlled trials?
- Intervention and control groups will be similar in all respects except the intervention minimising selection bias and confounding
- If participants are ‘blind’ to the treatment allocation, reporting bias is minimised
- Also, 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 causal relationship between the intervention and the outcome
- RCTs allow for multiple outcomes to be assessed
- RCTs allow for the incidence rate of the outcome to be measured
What are disadvantages of randomised controlled trials?
- Expensive to conduct as 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 can take up to years to do)
- Unequal at baseline
- Lack of blinding
- Loss to follow up
What is the difference between case-control and cohort study design?
Cohort:
1. Select population without disease and classify into exposed/unexposed
2. Measure disease incidence in exposed v unexposed
Case-control:
1. Select cases (diseased) and controls (undiseased)
2. Measure exposure in cases and controls
Define cross-sectional studies and their design?
They estimate frequency or outcome at a particular point in time
Either descriptive or analytical
DESCRIPTIVE = describe frequency of exposure in a defined population
ANALYTICAL = simultaneously collect information on both the outcome and potential risk factors in a defined population. Then compare the prevalence of the outcome in the people exposed to each risk factor with the prevalence in those not exposed
What are disadvantages of cross-sectional studies?
- Selection bias
- Information bias (recall bias)
- Measures prevalence rather than incidence cases which are of limited value for investigating aetiological relationships
- Can be difficult to establish the time-sequence of events in a cross-sectional study, the exposure may have occurred as a result of the outcome (reverse causality)
- Not good for studying rare diseases
- Unable to investigate the temporal relation between outcome and risk factors
What are the advantages of cross-sectional studies?
- Easy, relatively quick and economical
- No ethical difficulties
- Provides important information on the distribution of burden of exposures and outcome relationship
- Can be used as the first step in the study of a possible exposure-outcome relationship
- Data on all variables are only collected at one time point
- Multiple outcomes and exposures can be studied
- Easy for generating hypotheses
Write short notes on ecological fallacy?
- Ecological studies enable us to make ecological inferences about effects at a group level but do not enable us to make inferences about individual risk
- An ecological fallacy is a logical error which occurs when characteristics of a group are attributed to an individual
- Ecological fallacy can lead to a false or inaccurate conclusion about social phenomena and individuals within this social phenomena
- There are ways to avoid ecological fallacy including careful study design, statistical methods and critical thinking
- An example of when ecological fallacy occurs is in relation to break cancer and high fat consumption. It may be true that countries with higher levels of fat consumption have higher rates of breast cancer but that isnt to say that individuals who consume high contents of fat are more likely to have breast cancer
Write short notes on incidence risk and incidence rate
- There are two ways of measuring incidence: risk and rate
- Incidence risk is the number of new cases in interval divided by the population initially at risk
-Incidence rate is the number of new cases divided by the total person-time at risk - Which measurement to use depends on the study population. Incidence risk is good for static populations where as incidence rate is good for dynamic populations
- Incidence is different from prevalence as incidence describes the number of new cases in a defined period where as prevalence describes the number of existing cases
- Annual incidence describes the number of new cases in a calendar year
- Cumulative incidence refers to the frequency of new cases over a specific time period
Write short notes on prevalence?
- P = I X D (d being duration of disease)
- Prevalence is the number of existing cases in a population at a designated time
- Point prevalence is the proportion of people in a defined population that has the outcome under study at a specific point in time
- Period prevalence is the proportion of people in a defined population that has the outcome under study over a period of time
- Useful for assessing burden of disease within a population
- Valuable for planning
- Not useful for determining what caused the disease
