ARIMA Forecasting Accuracy

I am working on the revision to a manuscript in which I used Google Trends data to evaluate whether a particular event affected Google search patterns. I used an ARIMA model to forecast the expected search pattern had the event not occurred. The peer reviewer remarked that our study design did not control enough for confounding and asked that I provide the average accuracy of ARIMA projections in other time frames (previous year, previous months). I was hoping someone on this discussion board would be able to tell me the best way to address this comment. I appreciate your help.

This question may be better for the general stats board .