3. The computer-implemented method of claim 2 , wherein determining a quality score is a measure of user satisfaction that is dependent on user interactions with search results at the user device from which the first input sentence was received comprises, for each first input sentence: determining that the first input sentence was processed as a search query; determining a quality metric value for search results provided in response to the search query; and determining the quality score from the quality metric value.
3. The computer-implemented method of claim 2 , wherein determining a quality score is a measure of user satisfaction that is dependent on user interactions with search results at the user device from which the first input sentence was received comprises, for each first input sentence: determining that the first input sentence was processed as a search query; determining a quality metric value for search results provided in response to the search query; and determining the quality score from the quality metric value. 4. The computer-implemented method of claim 3 , wherein determining a quality metric value for search results provided in response to the search query comprises determining a measure of user satisfaction in response to the search results, the measure of user satisfaction being dependent on user interactions with the search results at the user device from which the input sentence was received.