Quasi Experimental Design
This is one type of experimental design that is very similar to the True Experimental Design with one key difference. If you recall, for an experimental design to be classified as a True Experimental Design, it must meet two criteria; 1) random assignment of participants to groups, and 2) manipulation of an internal variable (IV).
A Quasi-Experimental Design is exactly the same EXCEPT that there is no random assignment of participants to groups. That is the only difference between the two types of designs, but it is a very important difference. Without both random assignment and manipulation of an IV, a researcher can’t make cause and effect conclusions. Sometimes it just is not possible to randomly assign participants to groups.
An example of a quasi-experimental design would be a study in which you examine the effects of smoking on respiratory functioning. You might have people who smoke 1 pack a day and 2 pack a day smokers, but you can’t really assign them into these groups (is it ethical to make people who smoke 1 pack a day now smoke 2?) You would then run your study, but when you make conclusions, you can’t make any cause and effect conclusions.”