1. A computer-implemented method performed by data processing apparatus comprising one or more computers in data communication, the method comprising: receiving, by the data processing apparatus, web resource scores, each web resource score corresponding to a web resource and being a measure of a relevance of the web resource for a search operation; determining, for native applications, native application scores, each native application score corresponding to a native application and being a measure of a relevance of the native application for the search operation; determining a first proper subset of the native applications, each of the native applications in the first proper subset of native application being a native application determined to have a corresponding web resource for the native application; determining, from the native application scores of the native applications in the first proper subset and from the web resource scores of the corresponding web resources for the native applications in the first proper subset, a normalization factor that adjusts the native application scores to a notionally common scale; applying the normalization factor to the native application scores to generate normalized native application scores; and ranking the web resources and the native applications based on the web resource scores and the normalized native application scores, the ranking being indicative of the relevance of each web resource and native application for the search operation relative to each other web resource and native application.