What does this mean mathematically? This piece of text is an explanation of item-based recommendation by Mahout (mahout-item-based-recs) Can anyone explain the meaning of 3rd line (running average means?) - > for every item i that u has no preference for yet > > for every item j that u has a preference for compute a similarity s between i and j > > add u’s preference for j, weighted by s, to a running average > > return the top items, ranked by weighted average

A running average is also known as a moving average, which is often found in finance. For example, let's assume that we want to calculate the moving average for a six month sales period. This may be computed by taking the average of sales from January to June, then the average of sales from February to July, then of March to August and so on.

Moving averages reduce the impact of temporary variations in data and they help to show the data's trend more clearly. These types of average are known as "moving" or "running" as they are continually recomputed once new data becomes available, you move by dropping the earliest value and adding the latest value.

Questions of this sort would probably be better placed in the Cross validated stack exchange.