THE EFFECT SIZES IN A MEDIATION MODEL

Effect size is the amount of variance explained in the mediation model contributed by every single path. The researcher might be interested to know the relative contribution of individual paths in the mediation model, especially the effect of a mediator on the dependent variable. The researcher could assess the effect size for every path in the model, and also the mediated effect of the moderator variable itself. Specifically, the two effect sizes are:

(1) The R2 Measures the effect size of individual path in the model (3 paths)
(2) The R2 Measures the size of mediated effect of a mediator in the model
The range of values of R2 and its relative effect size based on definition by Cohen (1988) is given in Table 2.

1) R2 measures for the individual path in the mediation model is defined as follows
     i) r2XM represents the squared partial correlation between the X and M variables in the model,
     ii) r2XY denotes the squared partial correlation between X and Y variables in the model,
    iii) r2MY.X corresponds to the squared partial correlation between the M and Y variables with the influence of X removed,
 
For the above example - the following measures are obtained:
   i) The effect size of Skills & Training (X) on Job Attitude (M) is 0.78 (Figure 1)
  ii) The effect size of Skills & Training (X) on Career Advancement (Y) is 0.59 (Figure 2)
  iii) The effect size of Job Attitude (M) on Career Advancement (Y), controlling for X
This is equivalent to r2XMY - r2XY = 0.67 – 0.59 = 0.08 (Note: r2XMY=0.67 - Figure 1)
Conclusion: The effect size of a mediator on DV in small range

2) R2 Measures for the mediated effect of the mediator (Job Attitude).
This value is computed using the following formula:
R2MY – (R2 XMY - R2XY)
Where:
R2XY is the R2 for the model containing X and Y only (Figure 2)
R2MY is the R2 for the model containing M and Y only (Figure 3)
R2 XMY is the R2 for the model containing all X, M, and Y (Figure 1)
R2MY – (R2 XMY - R2XY)
0.66 – (0.67-0.59) = 0.58

Thus, based on Cohen (1988), the mediated effect size of the mediator Job Attitude in the relationship between Skills & Training and Career Advancement is in large range. 

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Date	:	18 & 19 January 2014 (Saturday & Sunday)
Venue	:	MPWS Training Centre, 63-1, 63-2, Jalan Kajang Impian 1/11, Taman Kajang Impian, Seksyen 7, 43650 Bandar Baru Bangi, Selangor
Time	:	8.30am – 5:30pm
Speaker	:	Associate Professor Dr. Zainudin Hj Awang
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