Experimental and Quasi-Experimental Research

 

What is the difference between the two?

 

A true experimental design uses a random sample (i.e., every member of the target population has an equal chance of being chosen to participate) and random assignment to experimental groups (i.e., each participant has an equal chance to be assigned to either to experimental or control group), whereas a quasi-experimental design lacks random assignment.

 

Example:

 

Foorman, B. R., Francis, D. J., Fletcher, J. M., & Schatschneider, C. (1998). The role of instruction in learning to read: Preventing reading failure in at-risk children. Journal of Educational Psychology, 90, 37-55.

 

Abstract [with notes added in red]:

 

 

First- and second-graders (N=285) receiving Title 1 services received 1 of 3 kinds of classroom reading programs: direct instruction in letter-sound correspondences practiced in decodable text (direct code); less direct instruction in systematic sound-spelling patterns embedded in connected text (embedded code); and implicit instruction in the alphabetic code while reading connected text (implicit code). [This is a quasi-experimental study, because the children were not randomly assigned to classes. The experimenters did not manipulate the enrollment in each class.] Children receiving direct code instruction improved in word reading at a faster rate and had higher word-recognition skills than those receiving implicit code instruction. Effects of instructional group on word recognition were moderated by initial levels of phonological processing and were most apparent in children with poorer initial phonological processing skills. Group differences in reading comprehension paralleled those for word recognition but were less robust. Groups did not differ in spelling achievement or in vocabulary growth. Results show advantages for reading instructional programs that emphasize explicit instruction in the alphabetic principle for at-risk children. [These results are answering causal questions; the type of instruction caused differences in outcome variables.]

 

Here are some selected graphs from the article:

 

edu-90-1-37-fig1a.png

 

edu-90-1-37-fig2a.png