Answer (a) — incorrect: Although it is not good to match controls to cases on too many factors, matching on fewer factors in itself does not guarantee that confounding is accurately controlled. It is best to match on the minimally necessary number of factors. However, matching on as few factors as possible may miss important sources of confounding that could be controlled in the design stage.

Answer (b) — incorrect: Matching on as many factors as possible is not a good strategy. It may be difficult and expensive to find controls for each case. It is not possible to determine which study was better at controlling for confounding by looking at the number of matched factors.

Answer (c) — correct: What should be of foremost importance when controlling confounding is that the variables are (1) confounders based on your theory of the exposure-disease relation, (2) meet the necessary conditions to be a confounder, (3) were measured properly and, (4) their effects were removed at the analysis stage. The reason Itoh et al. (2008) matched cases to controls on geographic location is because their source population for this study was very large and there were reasons to believe that background rates of breast cancer differed by geographical area. Teitelbaum et al.'s geographical area was limited to Nassau and Suffolk counties only.