How to quantitatively measure strength of friendship using observable factors? Is there any quantitative measure associated with the strength of a friendship or relationship? Ideally it should be able to be calculated based on observable factors, but should not require interaction with the subjects, just observation.

Note: Specifically I am analyzing fictionalized novels.
It's not my area, but a quick Google revealed this one.

The McGill Friendship Questionnaire - Respondent's affection (MFQ-RA). Item and validation information is available on the site.

It also mentions that it takes inspiration from  "the Friendship Questionnaire" and the "Friendship Qualities scale". So they might also be worth checking out.