This meta-analysis synthesizes research on media use in early childhood (0–6 years), word-learning, and vocabulary size. Multi-level analyses included 266 effect sizes from 63 studies (N total = 11,413) published between 1988–2022. Among samples with information about race/ethnicity (51%) and sex/gender (73%), most were majority White/Non-Hispanic and between 40%–60% female. Analyses revealed a small overall positive relation between screen media exposure and vocabulary (r = .23). Experimental studies yielded a small-to-medium effect (r = .30), with stronger effects for e-books than TV/video or games/apps, and non-significant effects for video chat. In correlational studies, there was no overall association between vocabulary size and naturalistic media exposure (r = .07), with the exception of naturalistic exposure to educational media (r = .17).

A crossover trial is an efficient trial design when there is no carry-over effect. To reduce the impact of the biological carry-over effect, a washout period is often designed. However, the carry-over effect remains an outstanding concern when a washout period is unethical or cannot sufficiently diminish the impact of the carry-over effect. The latter can occur in comparative effectiveness research, where the carry-over effect is often non-biological but behavioral. In this paper, we investigate the crossover design under a potential outcomes framework with and without the carry-over effect. We find that when the carry-over effect exists and satisfies a sign condition, the basic estimator underestimates the treatment effect, which does not inflate the type I error of one-sided tests but negatively impacts the power. This leads to a power trade-off between the crossover design and the parallel-group design, and we derive the condition under which the crossover design does not lead to type I error inflation and is still more powerful than the parallel-group design. We also develop covariate adjustment methods for crossover trials. We evaluate the performance of cross-over design and covariate adjustment using data from the MTN-034/REACH study.

Mendelian randomization (MR) addresses causal questions using genetic variants as instrumental variables. We propose a new MR method, G-Estimation under No Interaction with Unmeasured Selection (GENIUS)-MAny Weak Invalid IV, which simultaneously addresses the 2 salient challenges in MR: many weak instruments and widespread horizontal pleiotropy. Similar to MR-GENIUS, we use heteroscedasticity of the exposure to identify the treatment effect. We derive influence functions of the treatment effect, and then we construct a continuous updating estimator and establish its asymptotic properties under a many weak invalid instruments asymptotic regime by developing novel semiparametric theory. We also provide a measure of weak identification, an overidentification test, and a graphical diagnostic tool.