The absence of a means for obtaining confidence intervals for the impact of an
intervention represents a serious gap in the literature on the analysis of interrupted
time-series designs. In this article, an approximate procedure for constructing such
confidence intervals is developed both in general terms and for two specific forms of
intervention effects—abrupt temporary effects and gradual permanent effects. The
adequacy of the procedure is examined using Monte Carlo simulation. An illustration of
the procedure using a real data set is presented.
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