Julia Simner studied synesthesia, including its population prevalence. Sampling participants and checking consistency of reported associations helps distinguish an estimate based on a broader sample from one based only on people who identify themselves as synesthetic. Consistency checks provide evidence about particular experiences rather than one universal diagnostic test. [ref]
Synaesthesia: The prevalence of atypical cross-modal experiences by Julia Simner and others. 2006, sampling and objective consistency checks
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