A study of clinical disease wants to evaluate a new drug. There are three treatment groups: placebo, 30mg, 50mg.

Potential patients are screened to identify which patients have at least one parent who also had clinical disease.

100 patients are participated in each treatments group, 50 of whom had a parent clinical disease (and the other did not.) This is an example of a

block design

completely randomized design

matched pair design

cross-over design