when participants act differently because they know they are in an experiment

I expect that this is what the hypothesis is, so I will act this way.

The experimenter sees what he or she is looking for

Wanting to fit in; one acts differently or according to how others react

Variables that cannot be controlled, and may have affected the study

variables that the researcher should have controlled but did not

When somebody already knows what's going on

Makes progress; repeated practice makes participants better

Only get better becasue you think that you are being treated, something is helping you.

When the situation created is so unlikely to occur, that one has to wonder if there is any validity in the findings

tests what happens in the real world

neither the participants nor the person carrying out the experiment know what the aim, or which group is treated and which is controlled.

two different groups with different people

same group of people tested twice.