The anonymous commenter also wrote: The study you're referencing reviewed federal elections in one year. There were five incumbents running for office in primaries that year. The difference on the line or off was an average of 3 percentage points. From there we get a generalization that the reason no non-redistricted Congressional incumbent has lost in 52 years on the line. She used five elections for federal office involving incumbents from one year and then just decided to throw in a bunch of other elections that didn't involve incumbents or state legislative elections or mayoral elections or even a control group from another state.
The anonymous commenter also wrote: The study you're referencing reviewed federal elections in one year. There were five incumbents running for office in primaries that year. The difference on the line or off was an average of 3 percentage points. From there we get a generalization that the reason no non-redistricted Congressional incumbent has lost in 52 years on the line. She used five elections for federal office involving incumbents from one year and then just decided to throw in a bunch of other elections that didn't involve incumbents or state legislative elections or mayoral elections or even a control group from another state.