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.