1. A method performed by data processing apparatus, the method comprising: for each website of a plurality of websites, each website being a collection of resources served from a domain: obtaining, by a data processing apparatus, duration measurements of user device visits to resources of the website for a plurality of different users, each duration measurement being a duration of time that elapses between a user device requesting the resource and the user device requesting another resource; determining a website duration score that is proportional to duration measurements for the web site; obtaining, by the data processing apparatus, data describing a plurality of categories to which the website belongs; determining, by the data processing apparatus, for each category of the plurality of categories to which the website belongs, a category duration score for the category based on the website duration score for the website; and determining, by the data processing apparatus, from one or more of the category duration scores, a duration performance score for the web site, the duration performance score proportional to the one or more category duration scores from which the duration performance score is determined, the determining the duration performance score comprising, for the website: determining a first count of category duration scores that meet a threshold; determining a second count of a total number of category duration scores for the website; and determining the duration performance score based on the first count and the second count; wherein, for each website: the data describing a plurality of categories to which the website belongs includes, for each category, a category weight that is a measure of strength of association of the category to the website; and for each category, determining the category duration score further comprises determining the category duration score based on the category duration score and the category weight.
1. A method performed by data processing apparatus, the method comprising: for each website of a plurality of websites, each website being a collection of resources served from a domain: obtaining, by a data processing apparatus, duration measurements of user device visits to resources of the website for a plurality of different users, each duration measurement being a duration of time that elapses between a user device requesting the resource and the user device requesting another resource; determining a website duration score that is proportional to duration measurements for the web site; obtaining, by the data processing apparatus, data describing a plurality of categories to which the website belongs; determining, by the data processing apparatus, for each category of the plurality of categories to which the website belongs, a category duration score for the category based on the website duration score for the website; and determining, by the data processing apparatus, from one or more of the category duration scores, a duration performance score for the web site, the duration performance score proportional to the one or more category duration scores from which the duration performance score is determined, the determining the duration performance score comprising, for the website: determining a first count of category duration scores that meet a threshold; determining a second count of a total number of category duration scores for the website; and determining the duration performance score based on the first count and the second count; wherein, for each website: the data describing a plurality of categories to which the website belongs includes, for each category, a category weight that is a measure of strength of association of the category to the website; and for each category, determining the category duration score further comprises determining the category duration score based on the category duration score and the category weight. 6. The system of claim 1 , wherein determining the category duration score based on the category duration score and the category weight comprises determining a product of the category duration score and the category weight.