Independent Variables (IV) are the conditions within a study the experimenter directly manipulates. For example, in Osborn's (1996) "Makeup causes attractiveness?" study he assigned participants to see a woman with or without makeup. So makeup was the independent variable with two levels, makeup versus no-makeup. By comparing the average rating of attractiveness of the woman with makeup to the average rating by a different group of participants who saw the woman without makeup, he could estimate how important a cause makeup (independent variable) is in effecting attractiveness ratings (dependent variable).
So the cause is makeup and the effect is attractiveness ratings.
The participants who rated the woman shown with no makeup = the control group; different participants rated the same woman shown with makeup  = the experimental group.

We will be calling studies like this, where the experimenter can randomly assign each participant what group to be in, a true experiment. This is the most useful type of study since you can make unequivocal causal judgments from true experiments. When Osborn got a significant difference between the ratings of the woman in the makeup vs. the no-makeup condition he could say, "This shows that makeup causes higher attractiveness ratings." and there is no other reason why the results could have come out that way.

  The other major type of study is a descriptive/correlational study where the experimenter uses some pre-existing characteristic of the participants to form the groups. For example, Osborn & McCarthy (1987) showed that employees of  Bank A who could buy discounted bus tickets from Bank A rode the bus more than employees of  Bank B who had to pay full price for bus tickets. But Osborn could not unequivocally say, "This shows companies giving discounted bus tickets (independent variable) causes more bus ridership (dependent variable)." While this may likely be true there are alternative explanations (PRAH) for this difference besides Osborn's explanation. For example, a critic could point out people who are more likely to ride the bus anyway may be more likely to choose to work for Bank A (since they give a subsidy). So the critic says, "But it is not the subsidy that CAUSES them to ride the bus, they would ride it even if they did not get the subsidy. The type of people who ride buses also choose to work at places where there is a discounted ticket program." The only valid way to choose between these two explanations for greater bus ridership is to do more studies.    

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