Are Relational Inferences from Crowdsourced and Opt-in Samples Generalizable? Comparing Criminal Justice Attitudes in the GSS and Five Online Samples

Objectives: Similar to researchers in other disciplines, criminologists increasingly are using online crowdsourcing and opt-in panels for sampling, because of their low cost and convenience. However, online non-probability samples’ “fitness for use” will depend on the inference type and outcome variables of interest. Many studies use these samples to analyze relationships between variables. We explain how selection bias—when selection is a collider variable—and effect heterogeneity may undermine, respectively, the internal and external validity of relational inferences from crowdsourced and opt-in samples. We then examine whether such samples yield generalizable inferences about the correlates of criminal justice attitudes specifically. Methods: We compare multivariate regression results from five online non-probability samples drawn either from Amazon Mechanical Turk or an opt-in panel to those from the General Social Survey (GSS). The online samples include more than 4500 respondents nationally and four outcome variables measuring criminal justice attitudes. We estimate identical models for the online non-probability and GSS samples. Results: Regression coefficients in the online samples are normally in the same direction as the GSS coefficients, especially when they are statistically significant, but they differ considerably in magnitude; more than half (54%) fall outside the GSS’s 95% confidence interval. Conclusions: Online non-probability samples appear useful for estimating the direction but not the magnitude of relationships between variables, at least absent effective model-based adjustments. However, adjusting only for demographics, either through weighting or statistical control, is insufficient. We recommend that researchers conduct both a provisional generalizability check and a model-specification test before using these samples to make relational inferences.