HI Laura, please see responses below for your questions regarding HW3 key, mainly drafted by François. I also will be in the Epid kitchen today around 12:30 if you have follow up questions.
KEY #3: pg 2: confused by the terminology. in the paragraph with blue text after question 2, there is a discussion about primary measures of association. Should we really be calling it "cumulative incidence" if there is no person time in the measurement?
Comment: Please refer to Epi 203. In cohort study, there are two ways to measure incidence:
- Cumulative incidence. When comparing 2 groups, the ratio of 2 cumulative incidences gives the cumulative incidence ratio, often better known as risk ratio or relative risk
- Incidence rate. When comparing 2 groups, the ratio of 2 incidence rates gives the incidence rate ratio, also called relative rate
If there are no person time measurement, you can still measure the cumulative incidence.
last pg: part c. I don't follow why the statement "Association is not causation and patient X and W are not exchangeable." is the explanation to this question; |
Comment:
The thought process should be as follows:
- First, you calculated the true average causal effect (1.05) in b) because you were lucky enough to have access to the counterfactuals
- Second, in c) we ask you to imagine that you have access to only 1 scenario, which is what would happen in real life. You would therefore compute the observe association (0.7 in scenario 1 and 1.4 in scenario 2).
- Third, comparing your observed association to the true average causal effect, you realize that they are different and that it would have therefore been wrong to jump from association to causation in b), when you only had access to 1 scenario, I.e you performed a real study
- Last, you realize that this makes sense since association is not causation and that association measures should always go under high scrutiny before making causal claims. In particular, bias can arise from lack of exchangeability between patients X and W (since the outcome of X under treatment (or no treatment) cannot be used to represent the outcome of W under treatment (or no treatment) and vice versa)
We hope this helps!