Outlier detection for seasonal time series data

[Charlie Keyes]

I have 3 years of highly seasonal visit data by month to hospitals. I'm looking to detect outliers in this seasonal dataset, using an appropriate outlier detection method.

I've attempted to use the Simple algorithm for online outlier detection of a generic time series, however, this might not be suitable for highly seasonal data. Included below is a plot of the type of data I'm using and any suggestions on how to detect outliers in STATA would be most helpful.

Thank you in advance,

Charlie,