Nonequivalent Dependent Variables Study Discussion

Quasi-experimental, correlational and Naturalistic Observational Designs

Zajone, R. B., & Markus, G. B. (1075). Birth order and intellectual development

Psychological Review, 82, 74-88.

In this quasi-experimental study, Zajone and Markus used archival data to examine the relationship between birth order and intellectual development. According to Ray, in Chapter 10, a quasi-experimental study does not involve random assignment into experimental groups. Because birth order could not be randomly assigned, Zajone and Markus chose a quasi-experimental design. They found that intellectual ability decreased steadily from the first-born child, and as family size increased intellectual ability decreased. The investigators also found that first-born tended to be more intellectual than later borns; however, compared to later borns, first-born tended to be more anxious and neurotic. Later-born on the other hand, tended to be more social and affectionate. The youngest children tended to be more happy and creative.

Compared to true experimental designs, the findings in a quasi-experimental designs are more limited. Thus caution must be exercised by the investigators when interpreting results. Causal conclusions are not appropriate in studies that use a quasi-experimental approach. Therefore one may not conclude a fore mentioned study that birth order caused one be more or less intelligent.

Address the following questions in detail:

1. What kind of quasi-experimental design was illustrated in Zajone and Markus study?
2. Discuss the limitations of the design?