Hi On Wed, 26 Feb 2003, Hatcher, Joe wrote:
> I would argue that random assignment is only a means to > an end, the end being having at least two groups that are > assumed to be roughly equal on all variables. Seen that way, > a within-subjects design is simply another means of achieving > the same end. I understand the limitations, but would argue > that with appropriate counter-balancing these can be > overcome. This may be an argument of convenience for me or a > result of dissonance-reduction on my part; we have a very > small subject pool and strongly encourage within-subject > designs where possible I would agree with Joe here. Moreover, within-subject designs are certainly the dominant design in many areas of psychology that have a (perhaps undeserved according to Mike?) reputation as being experimental (e.g., cognitive, perception, ...). Within-subject designs also generally increase the power of the design/analysis, as the error term is usually smaller when individual differences in subjects are reduced. > > From: Mike Scoles > > I'm going to stick with Sir Fisher and reserve the term > "experiment" for > situations where there is random > assignment to conditions. I do not know > of > any > within-subjects designs that would not be better as mixed > designs. > Within-subject designs are too easily compromised > by history, maturation, > instrumentation, attrition, and > (sometimes) test sensitization and > regression issues. > Let's see, the only one of the "Big 7" that I left out > was > subject selection--the major problem with quasi-experiments. > Of > course, > good quasi-experiments can provide information > as useful as a marginal > experiment. If you think of it as random assignment of conditions to subjects (rather than the reverse) then within-subject designs can be accommodated. That is, it is possible make the conditions orthogonal to many (all?) potentially confounded variables in within-subject designs. I would also extend Mike's last statement even further. Non-experimental designs can also provide information as useful as (or more useful than) marginal or poorly designed experiments. Many students appear to believe (wrongly, I think) that an experiment necessarily means causal inferences are warranted, whereas in fact only well-designed experiments permit strong causal inferences. Best wishes Jim ============================================================================ James M. Clark (204) 786-9757 Department of Psychology (204) 774-4134 Fax University of Winnipeg 4L05D Winnipeg, Manitoba R3B 2E9 [EMAIL PROTECTED] CANADA http://www.uwinnipeg.ca/~clark ============================================================================ --- You are currently subscribed to tips as: [EMAIL PROTECTED] To unsubscribe send a blank email to [EMAIL PROTECTED]
