For an expected prevalence of chronic constipation of 20 percent, a sample size of 375 subjects was estimated to provide a 95 percent confidence interval (95%-CI) of +/- 4 percentage points. With an expected response of 75 percent, the global sample required totaled 500 subjects. Systematic, age-stratified sampling was carried out, arbitrarily considering three groups: 18-30 years, 31-50 years and 51-65 years.