Case control studies Flashcards
What is a case control study?
A case-control study is an epidemiological study that is often used to identify risk factors for a medical condition. This type of study compares a group of patients who have that condition with a group of patients that do not have it, and looks back in time to see how the characteristics of the two groups differ.
What is involved in a cohort study?
identify individuals with a disease (case)
identify similar individuals without the disease (control)
determine previous exposure
relate information on exposure to disease
What is the problem of case control studies?
source of cases and controls
Why is the source of cases important?
needs to representative of all people with the disease of interest
Why is the source of controls important?
needs to be in the same population as cases
if hospital based then ensure that they are not also related to exposure of interest
Give examples for a source of controls?
patients in hospitals for other conditions
visitors to hospital
GP practices
Why is matching required?
know about potential confounders e.g. age/gender
not interested in examining the association of these confounders
How is matching carry out?
matching means it is difficult to recruit controls
if using GP register then you can select by age and sex
What is undermatching?
cases and controls are not similar enough
e.g. age cases may be older and more likely to smoke which may have nothing to do with the disease being investigated
What is overmatching?
cases and controls may be too similar
e.g siblings
What bias can occur in case control studies?
recall bias
reverse causality
selection of cases
selection of controls
What is recall bias?
cases may remember more than controls
Recall bias is when a person’s recall of their exposure to a suspected disease risk factor could be influenced by the knowledge that they are now suffering from that particular disease. For example someone who has suffered a heart attack may recall having a highly stressed job. The stress that they now report experiencing may now be subtly different than the stress they would have reported at the time, before they developed the disease.
What is reverse causality?
disease may cause changes in recent exposures
e.g if diagnosed with lung cancer then the patient may stop smoking
What is a nested case control study?
A nested case-control study is a special type of case-control study in which ‘cases’ of a disease are drawn for the same cohort (population of people) as the controls to whom they are compared.
Use a big cohort and look at who develops the disease. Then select those who didn’t develop the disease (control) and look at the difference in the serum samples of those who developed the disease and the controls
What is the advantage of a nested case control study?
cheap, quick and easy
exposure is before the disease
What is the disadvantage of a nested case control study?
need cohort study with stored serum samples
What study can be used to investigate a risk factor for a rare disease?
case control
What study can be used to investigate a rare exposure?
cohort
What study can be used to investigate an exposure that may cause lots of different diseases?
cohort
What study can be used to investigate a risk factor for a disease quickly and cheaply?
case control
Recall bias is a problem in this study
case control
Can estimate risk and relative risk be calculated in a case control study?
No
What analysis can be used in case control studies?
Odds ratio
What is odds ratio?
Odds ratios compare the odds of the outcome in an exposed group with the odds of the same outcome in an unexposed group. Odds tell us how likely it is that an event will occur compared to the likelihood that the event will not happen.
Why can’t relative risk be used in a case control study?
Do not know the risk of the disease as you have started with cases with the disease
When can relative risk be used in a case control study?
if the disease is RARE then odds ratio is a good estimate of the relative risk
What is a cross sectional study?
This is an epidemiological study that describes characteristics of a population. It is ‘cross sectional’ because data is collected at one point in time and the relationships between characteristics are considered. Importantly, because this study doesn’t look at time trends, it can’t establish what causes what.
measure existing disease and current exposure
sample at 1 point in time without knowledge of disease or exposure
What is the advantage of a cross sectional study?
can look at exposures that won’t change e.g. gender
gives measures of prevalence and exposure rates
What is the disadvantage of a cross sectional study?
no use for rare exposures or rare diseases
not useful for causality
What study can be used to see if aspirin prevents heart disease?
clinical trial
What study can be used to see if aspirin causes stomach ulcers?
case control
What study can be used to see if exposure to blue asbestos causes lung cancer?
cohort
What study can be used to see if back pain is more common in men than women?
cross sectional survey
What are measures of importance?
absolute excess risk
attributable proportion
What is absolute excess risk?
risk in exposed - risk in unexposed
What is attributable proportion?
p(relative risk - 1) / 1 + p (relative risk - 1)
control group is representative of the population
