Until The Second Half Of Nineteenth Century  , Psychology Depended On Philosophy In Its Experiences Which Means It Depend On Individuals Feelings Toward Psychological Experiences And Rapid Observation . As For The Scientific Organized Experimentation And Observation By Machines Have Been Used Later   .

The Experimentation Looking For The Cause Of  Phenomena And How Is That Happened   Also Reveal The Causal Relationship Between Among Phenomena . The Experiences In Psychology Field Include In Its Simplest Form Only One Variable ( Effective) Which Is Called Independent Variable As Well As Another Variable ( Affected )   That Called  Dependent Variable .

So To Conduct  Any Experiment In Psychology We Need To A Sample And This Sample Should Be Represent The Studied Society That Mean We Can Generalize The Results Of The Study  On Society As A Whole  .

 Choosing Groups In Experimental Researches Is Considered  The First Step  In Experiences Success   . So The Researcher Will Focus On Displaying The Types Of These Groups And How To Design The Psychological Experiences With Their Advantage And Disadvantage.