1. A method comprising: receiving a sequence of one or more characters entered into a search field on a computing device, wherein the sequence of characters represents a first partial query; providing a first list of query completions for the first partial query for display on the computing device; receiving one or more additional characters entered into the search field upon display of the first list, wherein the sequence of characters and the additional characters cumulatively represent a second partial query; obtaining a second list of query completions for the second partial query; identifying, for each of one or more query completions in the second list, an initial ranking score for the query completion for the second partial query; identifying one or more query completions in the second list that appear in the first list; calculating demotion scores for the identified query completions, wherein a demotion score for a particular identified query completion is a function of both: the initial ranking score for the particular identified query completion, and a value that is based on a period of time between when the first list was displayed and when the additional characters were entered; using the demotion scores to demote one or more of the identified query completions to a lesser position within the second list, thereby forming a refined second list; and providing the refined second list of query completions for the second list of query completions for display on the computing device.