Researchers are generally required to report and interpret effect sizes and associated confidence intervals. When comparing two independent groups, the most commonly used estimator of effect size is Cohen’s ds where sample mean difference is divided by the pooled standard deviation. However, computing the pooled error term is not valid when both groups do not share common population variances. Furthermore, the assumption of equal population variances is unlikely in many psychological fields. Consequently, researchers shift to the use of Welch’s t-test over Student’s t-test in the context of hypothesis testing. Meanwhile, the question which effect size to report when equal variances are not assumed remains open. Based on Monte Carlo simulations, we compare Hedges’ gs (i.e. Cohen’s ds with correction for bias) to Glass’s gs, Shieh’s gs and Hedges’ g_s^*. Comparisons are made under normality as well as under realistic deviations from the assumptions of normality and equal variances. Although it is not directly related with Welch’s t-test (unlike Shieh’s gs), we recommend the use of Hedges’ g_s^* because it shows better properties than all other estimators. Practical recommendations, R package and Shiny App in order to compute effect size estimators and confidence intervals are provided.

21In 2016, the US Food and Drug Administration banned the use of specific microbicides 22 in some household and personal wash products. This decision was due to concerns that these 23 chemicals might induce antibiotic resistance or disrupt human microbial communities. Triclosan 24 and triclocarban (referred to as TCs) are the most common antimicrobials in household and 25 personal care products, but the extent to which TC exposure perturbs microbial communities in 26 humans, particularly during infant development, was unknown. We conducted a randomized 27 intervention of TC-containing household and personal care products during the first year 28 following birth to characterize whether TC exposure from wash products perturbs microbial 29 communities in mothers and their infants. Longitudinal survey of the intestinal microbiota using 30 16S ribosomal RNA amplicon sequencing showed that TC exposure from wash products did not 31 induce global reconstruction of either infant or maternal intestinal microbiotas following 10 32 months of exposure after birth. However, broadly antibiotic-resistant species from the phylum 33 Proteobacteria were enriched in stool samples from mothers in TC households only after the 34 introduction of triclosan-containing toothpaste. Despite the minimal effects of TC exposure from 35

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