I have to forecast some monthly timeseries 1.5 years into the future and then sum them up to get 3 semi-annual counts.
The forecasts give forecast intervals, but how do I get from the 6 monthly forcast intervals to one semi-annual one?
Is there some clear formula for that or a good estimate?
Or would it be better to sum the data into semi-annual counts first and then forecast those 3 periods into the future?
I'm using R and the forecast package.
Thanks in advance.
EDIT: My question has been marked as a duplicate of another question that is not actually similar.
First of all, the other questions deals with annual counts and I'm interested in semi-annual counts. It is not quite clear form the answer to the other question how I can get from the annual counts to semi-annual counts since the mechanism behind the trick mentioned isn't really explained.
Second, I'd like to know the theory behind it, not just a trick that works in R (we're using other time series platforms as well, so the R trick is of absolutely no help in those cases, even if it solves my problem with this particular aggregated forecast).
The Otexts book "Forecasting: Principles and Practice 2" seems to have a planned chapter about this problem, but there's only a headline and no text (unless it's my browser that's malfunctioning):