We collected data from 33 articles where summary statistics between biomolecule degradation and TSD of bloodstains were reported; however, effect sizes were only computed from 25 articles due to insufficient sample size in eight of the retained articles (Fig 1, Table ). A sample size below 4 cannot produce the required standard error based on Eq.2. Most studies reported more than one summary statistic; the 25 articles reported 109 effect sizes (Fisher's Zr) with a mean sample size of 9.7. R 2 was the most reported summary statistic, accounting for 70 of the 109, followed by correlation coefficients with 21 and p-values with 18. The mean effect size (Fisher's Zr) of our three-level meta-analytic model was 1.66 (r = 0.93, 95% CI: 0.88 -0.96, p-value <0.001; Fig. ); crudely, this means that 86 % of the variation is explained by time. The estimated between-study variance was 0.74 (I 2 = 74.8%), while the estimated within-study variance was 0.15 (I 2 = 14.9%). We noted that the three-level model provided a better fit than the two-level model which did not account for clustering (χ 2 = 63.04, p-value < 0.001, three-level: AIC = 208.4, BIC = 216.4; two-level: AIC = 269.4, BIC = 274.8). A total of 107 effect sizes were divided into three subgroups consisting of proteins, genetic material and whole blood and cellular elements. Whole blood and cellular elements had the highest effect size and correlation with TSD, followed by genetic material and proteins (Fig. , Table ), but no significant differences were found between subgroups (F = 0.35, p-value = 0.71). Although whole blood and cellular elements had the largest effect size, it also demonstrated the largest standard error, likely arising because of the small sample size (Fig. , Table ). For analytical techniques, 109 effect sizes were divided into 14 subgroups; mean effect sizes varied between techniques but no differences were found (F = 1.33, p-value = 0.21, Fig. ). Most techniques exhibited broad confidence intervals because of mean effect size computation using a small number of effect sizes (Table ). The red and grey dashed lines represent an effect size of 0 and the overall mean effect size of the meta-analysis, respectively. Note that the x-axis is scaled differently for A) than B-D). The pie chart is divided according to the number of summary statistics that contributed to each class of biomolecule; colours match those used in A).