There are many different ways of taking measurements in psychological research, one of these is the design method known as observations.
There are two type of observation, covert (meaning secretive) and overt (meaning out in the open)
Both of these have their disadvantages and advantages, for example, covert observations are very useful for eliminating what is known as the hawthorne effect. This effect states that when someone knows there behaviour is being observed, that behaviour changes. This means that the validity of the study is greatly improved as the study is very realistic, it occurs (usually) in a natural environment and is unaffected by the observation. It is however often difficult, if not impossible to view some behaviours in a truly covert way, e.g. childrens behaviour in the classroom, it is impossible to properly view children in their natural learning environment as the changes to the environment would be obvious for the children (imagine seeing a strange man sat in the doll house at the back of your classroom). Overt observations however, lack realism due to the hawthorne effect (and the fact there is usually a researcher there telling you what they’re looking for), however it does allow for greater manipulation of environmental factors and variables as the observation does not intend to investigate natural behaviours per se.