(This post is jointly written by the MetaLab team, with contributions from Christina Bergmann, Sho Tsuji, Alex Cristia, and me.)
A typical “ages and stages” ordering. Meta-analysis helps us do better.
Developmental psychologists often make statements of the form “babies do X at age Y.” But these “ages and stages” tidbits sometimes misrepresent a complex and messy research literature. In some cases, dozens of studies test children of different ages using different tasks and then declare success or failure based on a binary p < .05 criterion. Often only a handful of these studies – typically those published earliest or in the most prestigious journals – are used in reviews, textbooks, or summaries for the broader public. In medicine and other fields, it’s long been recognized that we can do better.
Meta-analysis (MA) is a toolkit of techniques for combining information across disparate studies into a single framework so that evidence can be synthesized objectively. The results of each study are transformed into a standardized effect size (like Cohen’s d) and are treated as a single data point for a meta-analysis. Each data point can be weighted to reflect a given study’s precision (which typically depends on sample size). These weighted data points are then combined into a meta-analytic regression to assess the evidential value of a given literature. Follow-up analyses can also look at moderators – factors influencing the overall effect – as well as issues like publication bias or p-hacking.* Developmentalists will often enter participant age as a moderator, since meta-analysis enables us to statistically assess how much effects for a specific ability increase as infants and children develop.
An example age-moderation relationship for studies of mutual exclusivity in early word learning.
Meta-analyses can be immensely informative – yet they are rarely used by researchers. One reason may be because it takes a bit of training to carry them out or even understand them. Additionally, MAs go out of date as new studies are published.
To facilitate developmental researchers’ access to up-to-date meta-analyses, we created MetaLab. MetaLab is a website that compiles MAs of phenomena in developmental psychology. The site has grown over the last two years from just a small handful of MAs to 15 at present, with data from more than 16,000 infants. The data from each MA are stored in a standardized format, allowing them to be downloaded, browsed, and explored using interactive visualizations. Because all analyses are dynamic, curators or interested users can add new data as the literature expands.