In experimental research, the researcher manipulates the variable and measures the variable.

If the study is not well controlled, it is said to have low validity. This happens when there are potentially other variables that influence the outcome of the experiment. 

In some experiments, one condition receives no treatment.  In other experiments, a substance with no known medical effects, such as sterile water, saline solution, or a sugar pill will be given to see if simply receiving a treatment will lead to a behaviour.  This substance is called a . 

One factor that influences experiments is - that is, where participants form an interpretation of the experiment's purpose and subconsciously change their behavior to fit that interpretation. 

A participant may fear being judged for their attitudes or answers to questions.  This may lead to effect. 

In addition, when studying health issues, it may be the case that an individual may respond positively to questions about their health, misjudging just how healthy their behaviours actually are.  This is known as bias.

One of the most important aspects of experiments is the operationalization of variables. If a concept or variable is not clearly defined, this can lead to problems with measurement - which means that the study may lack validity. 

Sometimes a study does not actually test what it claims to test - for example, it claims to be studying emotions, but actually is studying the effects of stress.  In this case, we would say that the study has poor validity. 

Often we see studies that have sampling bias.  In that case, we can say that the study has poor validity. Sometimes a study is culturally biased.  This means that the study has low cross-cultural validity because the study was in its approach. Finally, a lot of early psychological research was done exclusively with male participants - this research is considered to be

A problem with experiments is that if they are highly controlled and the environment is artificial, the study may have low validity. 

And sometimes a study which seemed important in the past is no longer considered relevant in modern society.  We could say that this study suffers from low validity. 

It is important that the procedure of an experiment is standardized.  This makes it possible to replicate the study and determine the of the findings.