Mediation analysis: interpret a prop. mediated 95% upper CI > 1

Question

I am using the R mediation package. The output of the mediate command is as follows:

Causal Mediation Analysis 

Quasi-Bayesian Confidence Intervals

                          Estimate 95% CI Lower 95% CI Upper p-value    
ACME (control)           -0.001633    -0.002128         0.00  <2e-16 ***
ACME (treated)           -0.001251    -0.002040         0.00  <2e-16 ***
ADE (control)            -0.001082    -0.003008         0.00   0.296    
ADE (treated)            -0.000700    -0.001937         0.00   0.296    
Total Effect             -0.002333    -0.003881         0.00   0.022 *  
Prop. Mediated (control)  0.677185     0.471999         1.66   0.022 *  
Prop. Mediated (treated)  0.496793     0.178559         2.01   0.022 *  
ACME (average)           -0.001442    -0.001940         0.00  <2e-16 ***
ADE (average)            -0.000891    -0.002488         0.00   0.296    
Prop. Mediated (average)  0.586989     0.329850         1.85   0.022 *  
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Sample Size Used: 19727 


Simulations: 1000 

As you can see the proportion mediated upper 95% CI interval for the treated group is 2.01.

How should I interpret this proportion greater than 1?

Answer 1

In the documentation for the mediate function you can read this:

the "proportions mediated", or the size of the average causal mediation effects relative to the total effect.

So, a proportion mediated that is greater than 1 would indicate that the size of the mediation effect is larger than the total. This could happen if the directions of the main and mediated effect were opposite. This is equivalent to suppression in ordinary regression. If the direct effect is very small, then the proportion mediated could be far greater than 1.

Then the question of whether there is mediation at all becomes kind of philosophical. If there is no effect, is there anything to mediate?