How could one compare depth of friendship of even two current friendships, let alone of current friendships with historical ones that are only accessible through people's mutable memories?

I'd assume this was measure previously, maybe im wrong, but it seems like something you could and would have been measured in the past.

But how can you measure except subjectively, and how can you compare subjective measurements when the whole point is that people's very idea of the meaning of phrases like "close friendship" have changed?

Its not my area, but apart from the the person subjective opinion there would be indirect ways to measure it. again I'm just guessing without looking around.