This paper reports a systematic review to address two objectives: 1, to compare the normative values of tests measuring fusional vergence; 2, to investigate sources of heterogeneity of prism accuracy. The review was conducted according to PRISMA guidelines with a Strobe checklist adapted to score the quality of evidence in 81 source papers. The results overviewed clinical implications, prism fusion range measured by step vergence, prism fusion range measure by smooth vergence, variation in method of assessment, target type, size and distance, order of assessment, encouragement during assessment, ocular dominance, refractive error and heterophoria, type of strabismus and severity, convergence insufficiency and symptoms, orthoptic / optometric treatment, reading and student populations. Sources of heterogeneity included age for the step vergence methods but not for the smooth vergence method. Maintenance of binocular function is dependent on multiple contributing factors with key elements including age, type and extent of ocular deviation.