Qualitative Individual Differences are Useful, but Reliability Should be Assessed and Not Assumed

Rouder and Haaf (2021) propose that studying qualitative individual differences would be a useful tool for researchers. I agree with their central message. I use this commentary to highlight examples from the literature where similar questions have been asked, and how researchers have addressed them with existing tools. I also observe that while the hierarchical Bayesian framework is a useful tool for studying individual differences, it does not relieve us of the requirement to evaluate the forms of reliability that are critical to our research questions.