HSPH1 Flashcards
Whats the difference between case control and cohort studies? [2]
cohort studies
* you start with a group of people (cohort), measure their exposures and follow them up to see who develops the disease.
case-control studies
* you start with a group of people with the disease (cases) and a group without (control) and compare their past exposures.
State 3 reasons why a case control study is better than a cohort study [3
Quicker to do usually – the follow up in some cohort studies will be long.
More practical if outcome is rare – (cohort would need many more patients)
Usually cheaper – fewer patients and quicker.
State 5 reasons when case control is worse than cohort [5]
May suffer from recall bias.
Different sorts of people (& their health) may be more likely to take part, depending on whether they are a case or a control, not true of a cohort where patients are usually all well at the start
Can only study one outcome (cohort studies can look at several outcomes)
May suffer from bias in selecting patients for cases and controls.
Cannot calculate absolute risks.
State a reason why cc or cohort may be conducted instead of a RCT? [1]
Both can be done where a randomised controlled trial is unethical.
Why can’t a clinical trial be used for this scenario? [1]
It is unethical to test something that may be potentially harmfull, let alone the ethics of giving people recreational drugs !
Why can’t a clinical trial be used for this scenario? [1]
It is unethical to test something that may be potentially harmfull, let alone the ethics of giving people recreational drugs !
State which of a case control or cohort study should have been conducted in this study and why
Case control: (compare past)
- It is more practical - the outcome is rare and some of the exposures reasonably common
- The controls reflect the population from which cases have arisen and are matched for age, race, ethnicity and neighbourhood, i.e. important confounders
Should a case-control or cohort study be used for this trial? Why [2]
Case control because:
- It is more practical - the outcome is rare and some of the exposures reasonably common.
- The controls reflect the population from which cases have arisen and are matched for age, race, ethnicity and neighbourhood, i.e. important confounders [B3]. The study demonstrates how hard it is to find matched controls.[B4]
(If controls are not representative of the population from which the cases have arisen this can lead to bias in the results and the [] could be wrong.)
(If controls are not representative of the population from which the cases have arisen this can lead to bias in the results and the odds ratios could be wrong.)
Define odds ratio [1]
An odds ratio is the ratio of the odds of the cases (those with the disease) being exposed compared with the odds of the controls (those without the disease) being exposed
When a disease is rare, the odds ratio is equivalent to which calculation? [1]
For rare diseases (i.e. diseases with an incidence or prevalence less than around 1%) the odds ratio is equivalent to the relative risk and can be interpreted as the ratio of the risk of developing the disease amongst an exposed person compared to an unexposed person.
Odds Marijuana use in cases = 113/26 = 4.35
Odds Marijuana use in controls = 222/70 = 3.17
Calculate the odds ratio [1]
Odds ratio Marijuana use in cases compared with controls = 4.35/3.17 = 1.37
Adjusted OR = 1.94 for smoking weed and testicular cancer. What does this mean? [1]
meaning that men that used Marijuana in this study had a 94% increased risk of developing testicular germ cell tumours after taking into account their other risk factors.
How what 95% confidence interval of 1.02 to 3.68 mean? [1]
The 95% confidence interval of 1.02 to 3.68 can be interpreted as meaning that we are 95% confident that the true increased risk of developing testicular germ cell tumours after taking into account their other risk factors lies between a 2% risk and a greater than 3 fold risk
What do crude (univariate or unadjusted) results mean? [1]
Crude (also called univariate or unadjusted) results, tell us the relationship between one risk factor and the outcome for everyone in the study.
