Using intervention time series analyses to assess the effects of imperfectly identifiable natural events: a general method and example

Background: Intervention time series analysis (ITSA) is an important method for analysing the
effect of sudden events on time series data. ITSA methods are quasi-experimental in nature and the
validity of modelling with these methods depends upon assumptions about the timing of the
intervention and the response of the process to it.
Method: This paper describes how to apply ITSA to analyse the impact of unplanned events on
time series when the timing of the event is not accurately known, and so the problems of ITSA
methods are magnified by uncertainty in the point of onset of the unplanned intervention.
Results: The methods are illustrated using the example of the Australian Heroin Shortage of 2001,
which provided an opportunity to study the health and social consequences of an abrupt change in
heroin availability in an environment of widespread harm reduction measures.
Conclusion: Application of these methods enables valuable insights about the consequences of
unplanned and poorly identified interventions while minimising the risk of spurious results.