1. A system, comprising: a data processing apparatus; and a non-transitory computer storage medium encoded with a computer program, the program comprising data processing apparatus instructions that when executed by the data processing apparatus cause the data processing apparatus to perform operations comprising: receiving query suggestion requests from a client device, each query suggestion request having been generated in response to a partial query input in a query input field of a search resource presented at the client device; in response to each query suggestion request: providing, to the client device, query suggestions responsive to the request; determining, for each query suggestion provided to the client device a quality measure for the query suggestion, the determining comprising: determining, for the query suggestion, a value indicating quality of resources referenced by search results responsive to the query suggestion, wherein a quality of each resource is determined by a valid prediction rate that is based on a ratio of number of times a search result referencing the resource was selected when provided in response to the query suggestion to a number of times a search result referencing the resource was provided in response to the query suggestion; determining if a prediction criterion is met, the prediction criterion being independent of a user selection of a query suggestion provided to the client device in response to one or more query suggestion requests and independent of receiving a completed query from the client device; in response to determining that the prediction criterion is met, providing search results to the client device, each of the search results identifying a particular resource that satisfies a query, and includes a resource locator for the resource, comprising: determining whether a query suggestion has a quality measure that meets a threshold; in response to determining that a query suggestion provided to the client device has a quality measure that meets the threshold, providing, to the client device, search results for the query suggestion having the quality measure that meets the threshold, the search results being responsive to the query suggestion; and in response to determining that none of the query suggestions provided to the client device have a quality measure that meets the threshold, providing, to the client device, search results for only the partial query, the search results being responsive to the partial query; and in response to determining that the prediction criterion is not met, not providing the search results to the client device.