In most cases where a moving average is taken of a time series the weights composing the average are constraints independent of time. For certain purposes, however, it is desirable to have weights which themselves reflect changing circumstances, as for instance in index numbers of prices where the quantities purchased may alter as time goes on. In such cases the weights themselves may be moving averages of time series and are said to be moving weights. More generally the term can be used to describe any set of weights which change with time.