Manipulation is simply the introduction of an element into a situation for some people, and not for others. Only through manipulation can there be clear and conclusive statements of cause and effect among the elements in the study. Depending on the outcome of the inferential statistics, we can say that our hypothesis is supported or not supported. The variables in the experimental design that the experimenter manipulates are the independent variables. These are the variables that researchers control and manipulate so that the phenomenon of interest is clearly and effectively studied.
Independent variables are independent of the participants in the study.
Study participants are not asked what condition they want to be in, rather they are assigned to a condition by the researcher. The dependent variables are what is measured, so dependent variables are the participants responses in the experiement. Those measures are dependent on the participants. Examples include test scores, percentage correct, and reaction time. The other factors in the experiment are the extraneous variables, which may have an effect on the study.