Calculate indirect effect, direct and total effects, proportion mediated, indirect-to-direct ratio, or a Sobel z test from mediation path coefficients.

Single-mediator linear additive model with compatible coefficient scales and no X × mediator interaction. Coefficients alone do not establish causation.

Describes the supplied coefficients; it does not rescale them.

Mediation Effect Size Formula

For a compatible single-mediator linear additive model, the estimated indirect effect can be calculated as the total coefficient minus the direct coefficient. This decomposition is not universal for nonlinear, interaction or differently scaled models.

ab = c - c'

Variables:

  • ab is the estimated indirect effect, equal to the product of paths a and b in the compatible linear model
  • c is the total effect of the independent variable on the dependent variable
  • c’ is the direct effect of the independent variable on the dependent variable after accounting for the mediator

Select Indirect effect from total and direct, then enter c and c’ on compatible scales. The calculator subtracts c’ from c. Alternatively, supply paths a and b and c’ in the product mode.

What is Mediation Effect Size?

The indirect effect summarizes a modeled association through a mediator. Coefficients alone do not establish a causal mechanism; interpretation also depends on study design, model specification and assumptions. Indirect divided by total is defined only for a nonzero total. Cancellation can produce negative ratios or values above 100%, so the ratio is not always a simple share explained. Sobel mode uses unstandardized coefficients with matching standard errors and the first-order normal approximation. It does not perform bootstrap inference; the product’s sampling distribution can make normal theory inaccurate, especially in small samples.

How to Calculate Mediation Effect Size?

The following steps outline how to calculate the Mediation Effect Size.

  1. First, determine the total effect (c) of the independent variable on the dependent variable.
  2. Next, determine the direct effect (c’) of the independent variable on the dependent variable after accounting for the mediator.
  3. Use the formula to calculate the mediation effect size (ab) = c – c’.
  4. Use coefficients from the same compatible model, observations and scale; do not mix standardized and unstandardized estimates.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Total effect (c) = 0.5

Direct effect (c’) = 0.3