1. A method performed by one or more computers, the method comprising: obtaining measurements of durations of user visits by multiple users to multiple resources included in a particular site, wherein the particular site is a collection of Internet resources, wherein the collection is those resources that are hosted on a particular server and can be accessed through a network using an Internet address corresponding to the particular server, or the collection is those resources in an Internet domain, or the collection is those resources in an Internet subdomain, or the collection is those resources in a subdirectory or an Internet domain or subdomain, and wherein the measurements comprise, for each of two or more resources in the particular site, measurements of a duration of time that elapsed between a user clicking on a search result link to the resource included in a search results web page and the user navigating back to the search results web page; and determining a site quality score for the particular site based at least in part on the plurality of measurements, wherein determining the site quality score for the particular site comprises computing a statistical measure from the plurality of measurements, and wherein the statistical measure is a measure of central tendency of the plurality of measurements.