What do you call a person who is not involved directly in a situation but they have some ideas and assumptions about it?

linguists!

We are conducting research on a suicide, and we are asking people who have suicidal ideation/behavior about the possible causes, but we're also asking those who don't have about what they think that may lead people to commit suicide. We're trying to make a comparison between outsiders' assumptions and reality. What term would you use to describe those "public outsiders"?

Thanks!

What term would you use to describe those "public outsiders"?

They can be known as "the control" or "the control group". These terms are more popular with experiments that use a drug on some and a placebo on others, but they also refer to a group who does not experience the experiment and is merely observed so as to discover natural behaviour/reaction.