To study the effect of cigarette smoking on platelet aggregation re-
searchers drew blood samples from 11 individuals before and after they smoked
a cigarette and measured the percentage of blood platelet aggregation. This study
can be found in Levine, P. H. (1973). An acute effect of cigarette smoking on platelet
function, Circulation, 48, 619-623.

The variables are:

  percentage of blood platelet aggregation before smoking
  percentage of blood platelet aggregation after smoking