Factors Affecting Power: Size of the Difference between Population Means (2 of 3)
In figure A, the null hypothesis that µdiff = 0 is true. The area shaded in black represents Type I errors; the striped area represents correct decisions not to reject the null hypothesis. The mean of the sampling distribution is µdiff which equals 0.
In figures B-D, the null hypothesis is false. The mean of each distribution is µdiff which is 10 for Figure B, 20 for Figure C and 30 for Figure D. The cutoff for significance is 19.6 and is the same for all figures. The area to the right of 19.6
is
shaded and represents power. The area below 19.6 is shaded 
and
represents Type II errors (the area below -19.6 is negligible). As µdiff
increases, power (the proportion of the distribution larger than
19.6) increases.