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Table 3 Worked examples for sample size calculations—based on choice of study design, existing literature on MCID and within- and between-subject SD using PD15 to inhaled mannitol as outcome measure

From: The use of the mannitol test as an outcome measure in asthma intervention studies: a review and practical recommendations

  Crossover study design—single PD15 measure Crossover study design –pre/post PD15 measures Parallel group design—pre/post PD15 measures
Recommended if Single dose of drug Short duration of intervention Longer duration of intervention
Outcome measure PD15 post-treatment Change in PD15 pre/post-treatment Change in PD15 pre/post-treatment
Within-subject SD* 0.8
SD* of within-subject difference 1.13
Between-subject SD* of differences 1.4
MCID 1 on DD scale
(i.e., doubling in PD15)
1 on DD scale
(i.e., doubling in PD15)
1 on DD scale
(i.e., doubling in PD15)
Total Sample size needed 13 23 64
  1. DD dose doubling, MCID minimum clinically important difference, PD15 provoking dose at 15% drop in FEV1; SD standard deviation
  2. *All SDs are on DD scale i.e., log2 transformed data; With a probability of a type-I error of 2-sided 5% and power of 80%