I'm dealing with sales forecasts (time-series with few data points) for a company on a department level (one company consists out of multiple departments). My approach is it to build a time-series model for each department in order to forecast sales for each department. My problem is, I also want to know the overall sales of the company. Beside the solution to just train a time-series model on the aggragated overall company sales data, I wonder if I also can just add up the predictions from each department in order to create an overall company sales forecast. If I can do so, what actually happens with the confidence intervals? Does the boundries of the confidence intervals for each department add up?