11. A computer-implemented method comprising: using one or more processors to perform operations of: receiving a search request; determining search results based on the search request, the search results comprising a first set of items in a first format and a second set of items in a second format, a number of items in the first set and the number of items in the second set reflecting a target ratio; and determining an order of presentation of the search results in a list of search results based on the target ratio, the order of presentation comprising a respective position of each of the search result within the list of search results, the determining comprising selecting a next item for positioning in the list of search results based on a desired exposure percentage associated with the first set of items in the first format.
19. The method of claim 11 , wherein the target ratio is associated with a target exposure percentage for items in the first format, the determining of the order of presentation of the search results comprising: determining a first new exposure percentage in the list of search results for items from the first set, assuming a next selected item is from the first set; determining a second new exposure percentage in the list of search results for items from the first set, assuming the next selected item is from the second set; comparing a first difference between the first new exposure percentage and the first target exposure percentage with a second difference between the second new exposure percentage and the second target exposure percentage; and selecting a next item for positioning in the list of search results from the first set or from the second set, based on a result of the comparing.