Most people are familiar with compressing data files so that they require less memory and they are easier to send electronically. Similar concepts are popular with process data historians. With process data, compression means reducing the number of data points that are stored, while trying to not affect the quality of the data. Compression can be accomplished using one of several algorithms (swinging door, Box Car Back Slope). Each algorithm uses some criteria to eliminate data between points where there is constant change (slope), within some tolerance.