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I have a dataset of the number of drug related events (e.g. arrest, non-fatal overdoses) a population of individuals have had. My aim is to see if a certain policy intervention that occurred on 1 June 2016 has had a significant impact on the number of drug related events that these individuals (approx. 4,000 people) have had. I thought of using interrupted time series analysis to see if there is a reduction in drug events for these individuals from the 2 years prior to the intervention compared to the two years after.

However, most (about 60%) of the individuals in this population have only had one drug event in the 2 years prior to the intervention. In fact, I have their data going back much longer than this for some – up to 10 years – and it seems these are rare/one off events for these people.
Using interrupted time series analysis on the 40% of individuals who have frequent events seems to make sense – they have the potential to show a decline e.g from a rate of 6 events to 4 events post intervention. However, for the 60% who have only one event, they don’t have much of an opportunity to show a decline post intervention. Even if these individuals did decline from 1 to 0 events in the 2 years post intervention, it seems like I would need to strongly qualify this finding with the fact they may have only had 1 event in their lifetime anyway, so no events over the following 2 years is hardly surprising.

Are my thoughts right about this? Should I not be using Interrupted TS? If not, is there another technique you could recommend to analyse the data?

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