The influence of age on sample size calculation in acute pain trials using morphine consumption as an end point

BACKGROUND: Many trials in acute postoperative pain use morphine consumption as an end point. Age has been shown to have a significant influence on morphine consumption but is rarely considered in end point analysis. METHODS: Simulation was used to investigate the effect of age on the power of various sample sizes to detect differences in morphine consumption in control and study groups. RESULTS: The sample sizes required for 80% power were approximately twice that predicted by a sample size program when comparing morphine consumption using a t test without adjustment for age. CONCLUSIONS: The model suggests that variations in age have a profound effect on sample size. Researchers may need to account for this to prevent both type 1 and type 2 error.