1. A computer-implemented method comprising: obtaining, by one or more data processing apparatus, from a data store, and over a network, visitor data collected from one or more client devices accessing, over the network, one or more remote servers that manage websites visited by the one or more client devices; generating, by the one or more data processing apparatus, multiple instances of a web analytics metric for a given website over a period of time based on the visitor data; identifying, by the one or more data processing apparatus, a change in a web analytics metric for the given website over the period of time based on a comparison of the multiple instances of the web analytics metric; determining, by the one or more data processing apparatus for each segment of two or more segments, a contribution score specifying a level of contribution to the change in the web analytics metric over the period of time that is attributed to a change in a value of the segment over the period of time, wherein the determination includes, for each segment: determining a first comparison between (i) a value of the web analytics metric at an earliest time in the period of time and (ii) a value of the segment at the earliest time in the period of time, determining a second comparison between (iii) a value of the web analytics metric at a latest time in the period of time and (iv) a value of the segment at the latest time in the period of time, adjusting the first comparison using a value of a weight time series of the segment at the earliest time, adjusting the second comparison using a value of the weight time series of the segment at the latest time, and computing the contribution score for the segment based on a difference between the adjusted first comparison and the adjusted second comparison.