Step 1: Understanding the Concept:
This question is testing whether you can identify a study design from its description, so the key is to notice which variable the researcher grouped subjects by first.
Step 2: Key Formula or Approach:
Ask which comes first in the description, the outcome or the exposure. If subjects are grouped by outcome, disease present or absent, and exposure is looked at afterward, that is a case-control design. If subjects are grouped by exposure and followed forward for the outcome, that is a cohort design.
Step 3: Detailed Explanation:
In this scenario, the two groups are defined by whether the woman was diagnosed with venous thrombosis or not, meaning the outcome is fixed first.
The researcher then looks backward at how many women in each group had been taking oral contraceptives.
This backward look from outcome to exposure is the defining feature of a case-control study.
A cohort study would instead start with exposed and unexposed women and follow both groups forward to see who later develops thrombosis, which is not what is described here.
A cross-sectional study would measure exposure and disease status together at one moment without splitting subjects by disease first, and ANOVA is only a statistical test, not a design at all.
Step 4: Final Answer:
This is a case-control study.