Effect of within-household reinfestation on design sensitivity.

The effect of within-household reinfestation on design sensitivity is investigated through statistical modeling of the reinfestation process. When reinfestation is present, household randomization tends to magnify treatment differences when compared with individual randomization or a randomized blocking design in the simple setting of two patients per household. The effect of reinfestation under more general household randomization settings is investigated by determining the relationship between the treatment effect and the number of patients per household. In an actual clinical study of treatment for head lice infestation, household randomization with proper stratification was adopted. The results from the study were consistent with theoretical expectations. This seeming contradiction to the blocking principle demonstrates the need to check whether all conditions are met before applying standard design principles.