Salaries of Members of Congress: Congressional Votes, 1990-2019 [Updated January 13, 2020] [open pdf - 1MB]
From the Introduction: "The automatic annual adjustment for Members of Congress is determined by a formula using a component of the Employment Cost Index (ECI), which measures rate of change in private sector pay. The adjustment automatically takes effect unless (1) Congress statutorily prohibits the adjustment; (2) Congress statutorily revises the adjustment; or (3) the annual base pay adjustment of General Schedule (GS) federal employees is established at a rate less than the scheduled increase for Members, in which case the percentage adjustment for Member pay is automatically lowered to match the percentage adjustment in GS base pay. Under the ECI formula, Members may not receive an annual pay adjustment greater than 5%."
CRS Report for Congress, 97-615
Congressional Research Service: https://crsreports.congress.gov/