Naturally, I applied a spreadsheet to this question when I was indexing the Dan Dare, Starlord and Tornado annuals, because they seemed to contain content that wasn't really related to the publication they were sprouting from.
I codified this as "if the content also exists in the parent publication, it's relevant".
Dan Dare Annual 1979 35% relevance
Dan Dare Annual 1980 35% relevance
Starlord Annual 1980 16% relevance
Starlord Annual 1981 26% relevance
Starlord Annual 1982 06% relevance
Tornado Annual 1980 18% relevance
Tornado Annual 1981 38% relevance