A meta-analytically derived nomological network of procrastination

1406 W. van Eerde / Personality and Individual Differences 35 (2003) 1401–1418
5. Affect: (a) state anxiety, including test anxiety and agitation;(b) depression, including
dejection.
6. Performance: self-reports or independent measures of (a) whether a deadline was missed or
not;(b) how much time was spent on preparing a task;(c) how much time was spent on
performing a task, such as an assignment;(d) grades obtained in a specific course;and (e)
grade point average.
2.3. Integration of effect sizes
The Rosenthal (1978, 1991) meta-analytic procedures were used to compare and combine
the effect sizes (for a comparison with two other meta-analytic approaches, see Johnson,
Mullen, & Salas, 1995). The correlations were transformed to Fisher’s zs and multiplied by
their degrees of freedom (n 3). These weighted Fisher’s zs were summed across studies within
each effect category and divided by the total degrees of freedom in the category to obtain the
average weighted Fisher’s z of each effect category. This Fisher’s z was transformed back into
r to obtain the weighted mean correlation for the effect category, and 95% confidence intervals
were computed for the weighted means. Subsequently, the w 2 for homogeneity of results
was computed within each effect category (Hedges & Olkin, 1985;Rosenthal, 1991) to establish
whether the findings can be interpreted as belonging to the same population of effect
sizes.
3. Results
After examination of the articles, the selection described above resulted in 88 articles, given in
the Appendix, from which I retrieved the effect sizes of 121 independent samples, with a total of
18,196 subjects. Sample sizes in the studies ranged from 32 to 618, with an average of 172. The
percentage of women in the samples ranged between 0 and 100%, with an average of 84%, indicating
that women were overrepresented in the studies.
The total range of ages in the samples ran from 16 to 43. In 60 samples, the age of the samples
was reported as mean or median. Averaging these statistics in the 60 samples resulted in a mean
age of 23, which indicates that the participants were relatively young.
The type of participants was recorded, to obtain an impression of their identity: 60 samples
consisted of college psychology students;29 were identified as college students with other majors
than psychology;seven of the samples included (high) school children;six contained non-student
adults;and in three samples the identity was not reported.
Many studies only mentioned one effect size, so that many of the categories contained
few correlations. The number of studies (k), total sample size per category, the average
results of each category weighted by the degrees of freedom of the total sample size in that
category, the 95% confidence interval around the average, and the homogeneity of the
findings, tested using a w 2 test, are presented in Table 1. The effect sizes can be considered
to be significantly different from zero when the confidence interval does not include the
zero.