Note that individual background variables such as sex and ethnicity do not satisfy the requirements of true experimental design since they cannot be purposively manipulated in this way (Practical assessment research and evaluation). True experiments are different from experimental design in that they are the only experiments that allow researchers to make causal conclusions based on study results and, therefore, provide greater internal validity (True experiments). It is only through random assignment that researchers can be assured that groups are truly comparable and that observed differences in outcomes are not the result of extraneous factors or pre-existing differences (Practical assessment research and evaluation). This means that the researcher needs so have control of the situation to have a reasonable chance of saying factors X and Y really affect outcome Z (True experiments). Internal validity is controlled in true experiments by assigning subjects to conditions randomly (i.e., only by chance can other variables be confounded with the independent variable) as well as controlling what, when, where, and how by controlling the way the experiment is conducted (True experiments - single-factor designs) through the use of control groups, random assignments to control and experimental groups and random assignments of groups to control and experimental conditions (True-experimental designs).

What are quasi-experimental designs? Why are they important? How are they different from experimental designs?

Practical assessment research and evaluation explains the quasi-experimental design. It consists of administering an outcome measure to two groups or to a program/treatment group and a comparison. For example, one group of students might receive reading instruction using one type of program while the other receives a different type of program. After twelve weeks, a reading comprehension test can determine which program was more effective.

Experimental research attempts to maintain control over all factors that may affect the result of an experiment so that a researcher can determine or predict what may occur. However, a quasi-experimental design does not meet all requirements necessary for controlling influences of extraneous variables (Practical assessment research and evaluation)....

In the student example above, the two groups might not be necessarily the same before any instruction takes place and may differ in important ways that influence what reading progress they are able to make. For instance, if it is found that the students in one of the groups perform better, there is no way of determining if they are better prepared or better readers even before the program and/or whether other factors are influential to their growth.
Still, quasi-experimental design is important because it is the best type of design often available for field studies when causal inference is desired (Quasi-experimental design). Explanations must be plausible given circumstances and consistent with data in situations where rival hypotheses has been shown to be implausible, but identification of what is and isn't plausible is not always apparent and interpretation issues must be carefully evaluated (Quasi-experimental design).

Bibliography

Comparative qualitative research methods. http://209.85.173.132/search?q=cache:3PulGqiG_ccJ:www.msu.edu/~lebas/Qual-Methods-2.ppt+single-case+and+small-N+research+designs&hl=en&ct=clnk&cd=6&gl=us

Cooper JO, Heron TE, Heward WL (2007). Applied Behavior Analysis (2nd ed. ed.). Prentice Hall.

ISBN 0-13-142113-1.

Descriptive and inferential statistics: Summary. http://www.habermas.org/stat2f98.htm

Kazdin. Wikipedia. http://en.wikipedia.org/wiki/Single_subject_design

Practical assessment research and evaluation http://pareonline.net/getvn.asp?v=5&n=14

Quasi-experimental design. http://209.85.173.132/search?q=cache:8PL0KDkAYPkJ:hcc.cc.gatech.edu/getDocument.php%3Fdoc%3D39+quasi-experimental+designs&hl=en&ct=clnk&cd=2&gl=us

Research methods. http://allpsych.com/researchmethods/trueexperimentaldesign.html

True experiments. http://www.wadsworth.com/psychology_d/templates/student_resources/workshops/res_methd/expermnts/expermnts_01.html

True-experimental designs. http://linguistics.byu.edu/faculty/henrichsenl/ResearchMethods/RM_2_08.html

True experiments - single-factor designs. http://axon.psyc.memphis.edu/~charlesblaha/3000/Lec%208%20True%20Exp/Lec%208%20True%20Exp%20-%20Part%201-STUDENTS.pdf