Leaving Congress: House of Representatives and Senate Departures Data, 1989-2020 [Updated April 20, 2022] [open pdf - 1MB]
From the Document: "In each Congress, Members leave the House or Senate for a variety of reasons. In the course of a Congress, those reasons could include resignation, death, or chamber action. At the conclusion of a Congress, Members depart due to retirement, electoral defeat, or pursuit of another office. In the 101st Congress (1989-1990) through 116th Congress (2019-2020), on average, two Senators and nine Members of the House of Representatives have left before the conclusion of a Congress. Over the same period, on average, 10 Senators (10% of Senate membership) and 59 Members of the House (13.4% of House membership) have left Congress upon expiration of their terms of office. This report provides data on Members who have left Congress between 1989 and 2020."
CRS Report for Congress, R41428
Congressional Research Service: https://crsreports.congress.gov/