Questions on Total Effect and Effect Size

[Yuyu Zhang]

Dear all,

In PLS, we use EFFECT frequently. Can anyone explain what is total effect, effect size and what are the differences between them? 

In my PLS result for a complex multi-direction PLS model, the sum of total effect in the model is more than 100%. Is it normal?

Thanks,

Yuyu

[Ned Kock]

Hi Yuyu.

 

Effect sizes are the absolute values of the individual contributions of the corresponding predictor latent variables to the R-square coefficients of the criterion latent variable in each latent variable block. With these effect sizes users can ascertain whether the effects indicated by path coefficients are small, medium, or large. The values usually recommended are 0.02, 0.15, and 0.35; respectively (Cohen, 1988). Values below 0.02 suggest effects that are too weak to be considered relevant from a practical point of view, even when the corresponding P values are statistically significant; a situation that may occur with large sample sizes.

 

Indirect and total effects are coefficients that are different from effect size coefficients. In fact, WarpPLS calculates effects size coefficients for each of the indirect and total effect coefficients reported.

 

In WarpPLS, indirect and total effects are calculated and shown, together with the corresponding P values, standard errors, and effect sizes. The calculation of indirect and total effects can be critical in the evaluation of downstream effects of latent variables that are mediated by other latent variables, especially in complex models with multiple mediating effects along concurrent paths. Indirect effects also allow for direct estimations, via resampling, of the P values associated with mediating effects that have traditionally relied on non-automated and thus time-consuming calculations based on linear (Preacher & Hayes, 2004) and nonlinear (Hayes & Preacher, 2010) assumptions.

 

Some useful videos:

 

- View Indirect and Total Effects in WarpPLS

    http://youtu.be/D9m4K_fv2vI

 

- Perform SEM Analysis and View Results with WarpPLS (Step 5)

    http://youtu.be/Srp9ewa1T8o

 

See WarpPLS User Manual for the references cited.

 

Regarding your question on the magnitude of total effects, they can be greater than 1, but that is not common and may be an indication of collinearity. Some of the path coefficients that make up a total effect of variable A on B (via intermediate paths) may be inflated due to collinearity – “bad” statistical suppression.

 

Ned

[Yuyu Zhang]

Thanks, Ned. 

Your responses were very informative and concise. 

Regarding the sum of total effects, except for the possible multi-collinearity, are there any other possible explanations to a summed total effects which is greater than 1? 

Yuyu

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