Ch13: Quasi-Experiments & Small N Designs

When are quasi-experiments used?

• When subjects can’t be randomly assigned to different treatment conditions

• When an IV can’t be manipulated

Ex post factor design (give an example)

experiments that are conducted after the groups have been formed

• Ex. An experiment is conducted after the subjects have become male/female

What is the problem with low internal validity?

• Lack of control

• Arises from confounds

• Can not draw a cause-effect conclusion

pre-test post-test design

one of the most frequently used quasi-experimental designs; used when we want to assess if occurrence of an event increases or decreases the existing level of behavior

• Ex. 01 X 02 = observed at time 1 (01); quasi-IV (X); observed at time 2 (02)

Nonequivalent control group designs (give an example)

A quasi-experiment that has at least one treatment group and one comparison group, in which participants have not been randomly assigned to the two groups, and in which at least one pretest and one posttest are administered

• Ex. 01 X 02 - 60 kids taking an SAT prep course; 01 -- 02 - 60 kids who didn’t take SAT prep course

  • Prep course group did better, but you can’t say it’s a cause-effect relationship because there wasn’t random assignment

time-series designs (give an example)

type of pre-test post-test design; measures DV on several occasions before and on several occasions after quasi-IV occurs; looks for changes in trend of data before and after treatment; aka simple interrupted time series design 

• Ex. 01 02 03 X 04 05 06

  • Several pre-test measures (01, 02, 03)

  • IV (X)

  • Several post-test measures (04, 05, 06)

What are some advantages and disadvantages of quasi-experimental research?

• Advantages: practical and gives research another perspective

• Disadvantages: no cause-effect and low internal validity