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The mediation package in R returns results in which:

  • Average Direct Effect (ADE) (the unmediated effect) is not statistically significant
  • The Average Causal Mediated Effect (ACME) (the effect of the mediator alone) is positive and statistically significant
  • The Total Effect (ADE+ACME) is larger than the ADE and ACME alone, positive and statistically significant

Does this indicate that the variable being mediated can only act through the mediator DV? I'm not quite certain as to how this would be interpreted.

Thank you for any help.

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This does suggest a "full" mediation, in which all of the IV's influence is mediated. The ACME being significant shows that the mediating process appears to be present. On the other hand, you don't have evidence that there is an ADE (insignificant result). The reason the total effect is larger than the ACME alone is that the estimate still includes the estimate for the ADE, however uncertain that estimate is for the ADE. The total effect has all the certainty of the ACME and the uncertainty of the ADE, but is still statistically significant because of the apparent strength of the ACME observation.

With that said, I would be careful in discussing these results since this analysis isn't designed to prove that there is no direct effect, but rather show that there is one. Your insignificant estimate for the ADE reflects a lack of evidence, not contrary evidence per se.

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