When designing clinical trials focused on functional constipation therapies, understanding the normative values of populations selected using the Rome III criteria is important for estimating baseline symptom severity, and for power analysis and sample size calculations. The objective of this review was to determine normative ranges for stool frequency and form in adults with functional constipation (Rome III criteria). Eligible studies reported stool frequency or form; random effects meta-analysis was performed with subgroup analyses to explore sources of heterogeneity. A total of 25 studies (43 groups, 2292 subjects) were included. Pooled estimates were 2.7 (95% CI 2.4-3.0) for weekly stools and 2.4 (95% CI 2.1-2.6) for stool form (Bristol scale). Heterogeneity was high for both outcomes (both I2=96%, P<0.001). Subgroup analysis revealed that weekly bowel movement frequency was higher in larger than in smaller studies (3.1 vs. 2.3, P<0.001) and in studies conducted in Europe compared with those in the Americas (3.1 vs. 2.2, P=0.02). For stool form, the use of a daily diary versus subject recall was the sole explanatory variable (2.5 vs. 2.1, P<0.05). We conclude that adults with functional constipation have significant variation in stool frequency and form, explained in part by geography and study design. [ABSTRACT FROM AUTHOR]