Table 5

Moderators of between-articles variance of true effects

ModeratorTypeNumber of effectsNumber of articlesP valueMarginal R2Between-articles variance
τ2I2Explained
Without moderators6441012.8%42.9%n.a.
U-87 MG sourcecat.6441010.075n.s.2.6%
U-87 MG authenticationcat.6441010.476n.s.2.8%
U-87 MG age
(cell passages)
cont.138110.238n.s.1.5%
Cell concentrationcont.113200.323n.s.3.8%
Confluence level at cell passagingcont.57110.319n.s.0.8%
Glucose level of culture mediumcat.6441010.0167.0%2.5%*40.1%10.9%
Mycoplasma exclusioncat.6441010.491n.s.2.8%
Supplemented antibioticscat.6441010.094n.s.2.6%
FBS sourcecat.6441010.067n.s.2.6%
Type of untreated controlcat.6441010.370n.s.2.8%
Articles reporting qualityint.6441010.0313.3%2.6%†41.7%5.0%
TMZ conc.cont.644101<0.00138.6%3.4%‡64.3%0.0%
Treatment durationcont.644101<0.0016.0%2.9%§45.7%0.0%
  • Random-effects three-level meta-regressions with the raw data the effects were calculated with as first level, the reported effects as second level and the articles the effects were reported in as third level. Marginal R2 indicates the regression model fit69 ; τ2: estimator of the variance of true effects; τ: square root of τ2; I2: proportion of between-articles variance of the total observed variance, including sampling error. τ2 estimator: restricted-maximum likelihood. The column ‘Explained’ indicates the reduction of τ2 after including the particular moderator compared with τ2 without moderators (only applicable if the number of included effects and articles is identical). For some continuous moderators, the number of effects and articles included in the regression is reduced due to non-reporting, which leads to a limited comparability of τ2 between parameters with different numbers of belonging articles and effects. ‘Not reported’ was included as a category for categorical moderators. P value is given for the test of the moderator. R2, I2 and the explained heterogeneity were only calculated for moderators that prove significance in the test of the moderator (α=0.05).

  • *95% CI of τ2: 1.7% to 3.6%.

  • †95% CI of τ2: 1.8% to 3.8%.

  • ‡95% CI of τ2: 2.5% to 4.8%.

  • §95% CI of τ2: 2.1% to 4.2%.

  • cat., categorical; cont., continuous; FBS, fetal bovine serum; int., interval; n.a., not applicable; n.s., not significant; TMZ conc., temozolomide concentration; U-87 MG, Uppsala-87 Malignant Glioma.