Question: An enterprising practitioner of yoga contends that yoga has improved her patients’ flexibility. To prove her clam she cites data obtained from 15 patients who completed a felxibilty scale before and after yoga. By subtracting their function prior to treatment from their level after treatment she was able to conclude yoga provided up to a 40% increase in flexibility. Her scale was:

0=poor flexibility

1=slight flexibility

2=some flexibility

3=average flexibility

4=excellent flexibility

Is her claim credible? Please explain why.