Model 1: M = a*X + intercept

Model 2: Y = b*M + c’*X + intercept

Model 3: Y = c*X + intercept

Total effect c = a*b + c’

If indirect effect a*b is statistically significant, then mediation has occurred

Percent mediated: PM = ab / (ab + c’)

Link to SAS code to calculate a*b and PM with bootstrapped confidence intervals, illustrated using example data from Mackinnon’s “Introduction to Mediation”.