Background: To illustrate the advantages of using network meta-analysis (NMA) as compared to a trial or a pairwise meta-analysis to estimate the amount of missing outcome data (MOD) for a target comparison in order to adjust the required sample size for possible participant losses in a future trial.
Methods: We introduced the concept of transitive risks to obtain the absolute risks of MOD for all interventions of the network. We used the network of a published systematic review on a binary outcome to apply the proposed concept and to calculate the required sample size in a future trial for a selected target comparison. For that comparison, we also calculated the required sample size using the corresponding trials separately, and after pooling these trials in a random-effects meta-analysis.
Results: Ignoring MOD from the sample size calculation led to the smallest sample size. When either trial was considered, the risk of MOD ranged from 1% to 13% in the compared intervention arms, therefore, increasing the sample size from 1% to 12%. Performing a pairwise meta-analysis yielded a risk of MOD equal to 6% and 9% in the active and control arms, respectively, which inflated the sample size by 8%. Using NMA, the corresponding risks of MOD were 10% and 13%, which increased the sample size by 13%.
Conclusions: Provided that the transitivity assumption holds, incorporating the absolute risks of MOD in the sample size calculation for a target comparison of the network led to better planning of a future trial.