There are several differences between informal and formal observation, with one of the main differences being that informal observation is unstructured and unobtrusive. Additionally, informal observation may also be a precursor to formal observation, and carried out when the experimenter knows little about the group being observed.
Formal observation is typically carried out in the form of a survey or a similar way of collecting results from an audience. In formal observation, respondents will reply to the same series of questions. Informal observation is often conducted when respondents may not be aware, perhaps in public places such as shops or malls.