The default order of results differs between the WorldCat.org interface and the FirstSearch interface. Results are displayed according to relevance in WorldCat.org, so the items most closely related to a user's search terms will appear at the top of the results. In the FirstSearch view of WorldCat, results are displayed according to the number of holdings for each item. Therefore, items that are owned by the most WorldCat libraries will appear at the top of the results.
In addition, WorldCat.org results cluster related editions and formats under a representative record, while FirstSearch results display a separate record for each edition or format.
As with major search engines, results for the same search on WorldCat.org may vary over time, as the WorldCat database grows and OCLC improves its supporting algorithms.