Deriving dichotomous outcome measures from continuous data in randomised controlled trials of analgesics: Verification from independent data

A previously established relationship for deriving dichotomous from continuous information in randomised controlled trials (RCTs) of analgesics has been tested using an independent data set. Individual patient information from 18 RCTs of parallel-group design in acute postoperative pain (after abdominal, gynaecological and oral surgery) was used to calculate the percentage of the maximum possible pain relief score (%maxTOTPAR) and the proportion of patients with >50%maxTOTPAR for the different treatments, The relationship between the measures was investigated in 85 treatments with over 3400 patients. In 80 of 85 treatments (94%) agreement between calculated and actual number of patients with >50%maxTOTPAR was within four patients per treatment and in 72 (85%) was within three (average of 40 patients per treatment, range 21-58 patients). Summing the positive and negative differences between actual and calculated numbers of patients with >50%maxTOTPAR gave an average difference of 0.30 patients per treatment arm. Reports of RCTs of analgesics frequently describe results of studies in the form of mean derived indices, rather than using discontinuous events, such as number or proportion of patients with 50% pain relief. Because mean data inadequately describe information with a non-normal distribution, combining mean data in systematic reviews may compromise the results. Showing that dichotomous data can reliably be derived from mean data in acute pain studies enables data published as means to be used for quantitative systematic reviews which require data in dichotomous form.