where the parameter formula_22 quantifies the degree to which "M" modifies the effect of "X" on "Y". Even when all parameters are estimated from data, it is still not obvious what combinations of parameters measure the direct and indirect effect of "X" on "Y", or, more practically, how to assess the fraction of the total effect formula_23 that is "explained" by mediation and the fraction of formula_23 that is "owed" to mediation. In linear analysis, the former fraction is captured by the product formula_25, the latter by the difference formula_26, and the two quantities coincide. In the presence of interaction, however, each fraction demands a separate analysis, as dictated by the Mediation Formula, which yields: