least formal and constrained by fewest rules. provide foundations of biological and social sciences: correlation studies observation in nature, but involve more formal measurement, of environmental events of individuals physical and social characteristics, and of their behaviour. examination of relations between 2+ measurements of behavior or other characteristics of people/animals: experiments go beyond measurement; make something happen and observation results. only thing that can positively identify the causal relations among events. study where independent variable can be changed and observe whether manipulation affects value of dependant variable. only thing that can confirm the existence of cause-and-effect relations among variables. provide general accounts of phenomena stumbled upon y steps of the scientific method: identify the problem and formulate hypothetical cause-and-effect relations among variable, design the experiment. independent variable must be controlled so no other variable is responsible for any changes in the dependant variable: perform the experiment.