Resolution:
## Figure 2.
Power contour graphs for several intraclass correlation coefficients (ICCs) and effect
sizes*. Effect size is presented in columns and ICC in rows. In situations above or
to the right of the red curve, the statistical power is greater than 90%. In situations
between the red and blue curves, the statistical power is between 80% and 90%. In
situations between the blue and red curves, the statistical power is between 60% and
80%. For vertical curves, increasing the cluster size is pointless. The number of
subjects required, assuming individual randomization, is 24 to achieve a power of
40% to detect an effect size of 0.50, and 38, 28, 18 and 11 to achieve powers of 90%,
80%, 60% and 40%, respectively, to detect an effect size of 0.75, thus, the reason
why curves are truncated. *Effect size = absolute difference between the two intervention-specific
means divided by the S.D. of the response variable.
Guittet |