Using aggregate data (e.g. monthly) when data on a finer scale (e.g. daily) is available.
In digital signal processing, downsampling is the process of resampling in a multi-rate digital signal processing system. Downsampling can be synonymous with compression or describe an entire process of bandwidth reduction (filtering) and sample-rate reduction. When the process is performed on a sequence of samples of a signal or other continuous function, it produces an approximation of the sequence that would have been obtained by sampling the signal at a lower rate (or density, as in the case of a photograph).
Based on the article "Downsampling (signal processing)" in Wikipedia.