Zar (1999) describes a setting in which it's desired to compare two different drugs to see which drug makes blood clot faster in samples taken from 12 people with hemophilia (one sample of blood per person) who agreed to be part of the study. here are three possible experimental designs for this situation. 6 of the people are assigned to drug d and 6 to drug g at random. the number of years the person's body has been unable to produce a normal amount of blood coagulation (a measure of severity of illness) is identified as a potential confounding factor (pcf), and the 12 people are grouped into 6 pairs in such a way that within each pair both people have the same value of the pcf; the assignment to drug b or g within pairs is done at random. the time to clotting is measured for all 12 people on four separate occasions; 6 of the people (chosen at random) are observed on the schedule.

(1) measure clotting time.

(2) administer drug d for an appropriate amount of time

(3) measure clotting time

(4) wait an appropriate time until all effects of the drug have disappeared

(5) measure clotting time

(6) administer drug g for an appropriate amount of time

(7) measure clotting time and the other 6 are treated similarly except that drug g is given first followed by drug b.

identify which, if any, of these designs can be described by the following terms: completely randomized, randomized-blocks, matched-pairs, repeated measures.

then briefly discuss which, if any, of these designs are valid for arriving at correct causal conclusions about the comparative effects of the two drugs, and (if any of them are valid) which are likely to be more accurate at estimating those effects.