I think one good discriminant is: Would you want to explain it candidly? E.g.: "Starting next week, we'll try to affect your mood as part of an experiment". Or

I think one good discriminant is: Would you want to explain it candidly? E.g.: "Starting next week, we'll try to affect your mood as part of an experiment". Or "the new version of our product purposefully tries to depress you a bit" followed by "we didn't like the results of it so we've changed it back".