Adult stuttering prevalence I: Systematic review and identification of stuttering in large populations.

Purpose: Stuttering epidemiology is reviewed with a primary goal of appraising methods used to identify stuttering in large populations. Secondary goals were to establish a best estimate of adult stuttering prevalence; identify data that could subgroup stuttering based upon childhood versus adult onset and covert versus over behaviour; and conduct a preliminary assessment of the degree to which stuttering features as a co-occurring diagnosis.

Methods: Systematic review followed PRISMA guidelines. Quality assessment was based on the Joanna Briggs Institute Prevalence Critical Appraisal Tool, with criteria adjusted for appraisal of stuttering.

Results: 15 sets of data were assessed for quality, with three meeting criteria for inclusion. These estimated adult stuttering prevalence at 0.67% at age 14-17 years (Taghipour et al., 2013); 0.21% at age 16-20 years (Tsur et al., 2021); and 0.63% when aged over 21 years (Craig et al., 2002).

Conclusion: Systematic review indicates adult stuttering prevalence is between 0.6-0.7%. A false positive paradox follows from the low prevalence of stuttering in the general population, creating a need for very high specificity when measuring stuttering in the general population. Failure to achieve high specificity (99.9% is suggested) leads to loss of statistical power due to presence of false positives. A corollary of the false positive paradox is that sensitivity in measurement of stuttering can be relatively low (90% is suggested) before general population estimates of stuttering prevalence are appreciably affected. Despite this relaxation of measurement requirements regarding sensitivity, covert stuttering is likely to have been underestimated. Covert stuttering might be accounted for using data from prospective cohort studies, however such a revision seems unlikely to exceed the widely-accepted 1% adult stuttering prevalence estimate; see Gattie, Lieven & Kluk (2024 this issue) for an estimate at 0.96 %. When used to estimate stuttering prevalence, data reported by Tsur et al. (2021) are outlying, with the relatively low estimate possibly due to origin as military conscript data and/or generalised healthcare screening.