1. A method, comprising: receiving, at one or more computing devices, information identifying a first knowledge element from a plurality of knowledge elements; for each of a plurality of similar knowledge elements: determining, by the one or more computing devices, a first value based on actions taken by a user with respect to a respective knowledge element from the plurality of similar knowledge elements, determining a second value that represents similarity of the first knowledge element to the respective knowledge element, and determining an expertise component value for the respective knowledge element based at least in part on a multiplicative product of the first value and the second value; and determining, by the one or more computing devices, an expertise value that represents the user's expertise level relative to a subject matter described by the first knowledge element based on the expertise component values for the plurality of similar knowledge elements.