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Examination of Mediating Variables

Richard Chambers

Hypothesis 6a predicted that improvements in executive function would mediate the benefits of mindfulness practice on both metacognitive processing and psychological wellbeing. In order to test this, a number of mediating relationships were examined to ascertain whether they met the criteria of Baron & Kenny (1986; see Method section). Table 5 indicates which of the criteria each hypothesised relationship met.

Table 5

Stepwise Assessment of Adherence to the Criteria of Baron & Kenny (1986), for each Hypothesised Mediating Relationship between Independent Variables (IV), Mediating Variables (MV) and Dependent Variables (DV)


Criterion
Proposed Mediating Relationship (IVMVDV) Correlation between IV and DV Correlation between MV and IV Correlation between MV and DV Partial correlation between IV and DV when MV is controlled for in linear regression Met all criteria?
GroupDSBBDI .45** -.41** -.18 .42 *
GroupDSBBAI .05 -.41** -.24T .-.06 *
GroupDSBPANAS+ -.03 -.41** .22T .06
GroupDSBPANAS- .32* -.41** -.09 .31
GroupDSBRRS .23T -.41** -.10 .21
GroupDSBMAAS -.5*** -.41** .31* -.43 ** yes
GroupNeutral Switch Task BDI .45** .34* .41** .34 * yes
GroupNeutral Switch Task BAI .05 .34* -.05 .07
GroupNeutral Switch Task PANAS+ -.03 .34* -.07 .002
GroupNeutral Switch Task PANAS- .32* .34* .09 .32*
GroupNeutral Switch Task RRS .23T .34* .09 .23
GroupNeutral Switch Task MAAS .45** .34* -.08 -.54***
GroupAffective Switch Task BDI .05 .20 .27T .41**
Group Affective Switch Task BAI -.03 .20 -.07 .06
Group Affective Switch Task PANAS+ .32* .20 .10 -.05
Group Affective Switch Task PANAS- .23T .20 .11 .31*
Group Affective Switch Task RRS -.5*** .20 .06 .24
Group Affective Switch Task MAAS .001 .20 -.21 -.51***
GroupOverall Switch Task BDI .002 .32* .39* .36* yes
Group Overall Switch Task BAI .39 .32* -.05 .07
Group Overall Switch Task PANAS+ .42 .32* .04 -.04
Group Overall Switch Task PANAS- .02 .32* .12 .31
Group Overall Switch Task RRS .07 .32* .08 .24
Group Overall Switch Task MAAS .001 .32* -.15 -.51

Note: Values provided Pearson r-values of a bivariate correlation between residual measures of affect, metacognitive processing, and executive function, except for column 4, which represents Pearson r-values of a partial correlation between the same measures. All four r-values must be significant for valid calculation of a mediating relationship.
*p < .05 **p < .01 ***p < .001 Tp < .1 (trend).

As can be seen from Table 5, all conditions were satisfied for the calculation of three mediation tests, each of which demonstrated: (1) a significant correlation between the IV and DV; (2) a significant correlation between the MV and IV; (3) a significant correlation between the MV and DV; and (4) a reduced correlation between the IV and DV when the MV is controlled for. The following are results of Sobel tests (reporting Goodman I values) for each of these mediating relationships:

1. Increased scores on the DSB mediated the relationship between group membership and improved scores on the MAAS (z = 2.06, p = .04).

2. There was a strong trend towards decreased RTs on the neutral condition of the IST mediating the relationship between group membership and improved scores on the BDI (z = 1.77, p = .08).

3. Decreased overall RTs on the IST (collapsed across neutral and affective) were not found to mediate the relationship between group membership and improved scores on the BDI (z = 1.54, p = .12).

Note that the statistics provided by Sobel tests are estimates, and so should be interpreted with caution (Preacher & Leonardelli, 2001).

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