  N-of-1 trials, single participant trials in which multiple treatments are
sequentially randomized over the study period, can give direct estimates of
individual-specific treatment effects. Combining n-of-1 trials gives extra
information for estimating the population average treatment effect compared
with randomized controlled trials and increases precision for
individual-specific treatment effect estimates. In this paper, we present a
procedure for designing n-of-1 trials. We formally define the design components
for determining the sample size of a series of n-of-1 trials, present models
for analyzing these trials and use them to derive the sample size formula for
estimating the population average treatment effect and the standard error of
the individual-specific treatment effect estimates. We recommend first finding
the possible designs that will satisfy the power requirement for estimating the
population average treatment effect and then, if of interest, finalizing the
design to also satisfy the standard error requirements for the
individual-specific treatment effect estimates. The procedure is implemented
and illustrated in the paper and through a Shiny app.
