In psychiatry, an informant is someone who can report on a case without causing observer effects.
[1] For example, informants may be family members or friends of a psychiatric patient.
In such cases, it is necessary to interview multiple informants and combine the data.
[2] Further compounding the difficulty, informants may also suffer from conditions themselves, which makes them more likely to describe others as having the informant's condition.
It is not clear whether this makes them more or less objective than healthy informants.