Retirement slump refers to the average falloff in the party's vote when the incumbent retires. A positive value of the sophomore surge represents an incumbency advantage. The retirement slump should be positive for an incumbency advantage to exist.[1]
Sophomore surge is the average vote gain for freshman winners in election 1 who run again in election 2. Retirement slump is the average vote loss for the parties whose candidates won election 1 and did not run in election 2.[2]
When a Sophomore surge and a Retirement slump combine, it is what is called a slurge.