This is the latest item in the SS series, the most recent of which was SS-17. This series is an annual data publication. SS-18 contains the most recent year's data; however, it does not exactly render previous versions obsolete, since readers often reference older versions to determine how the data has changed over time. This publication consists of a PDF (allowing the reader to see the data in its original form) and an Excel spreadsheet (allowing the reader to perform their own analysis of the data).