Dear Listers,
I've been out of the loop for quite a while, but here's an interesting
issue I don't think I've ever seen addressed.
I have a client who intended to use a wait-list control design --
participants were to wait 6 months or so, then get a 6-month
treatment. For various reasons, the participants ended up being 8
wait-list only, 12 treatment-only, and 4 both. That is, if we
pretend that all participants were on the same timeline, it would look
like:
Pre-wait testing ---- Post-wait/pre-treatment testing -----
Post-treatment testing. But only 4 participants have all 3
testings.
Because of the very small N, I want to squeeze the maximum info out of
the data. What we have is a sort of combined repeated-measures and
between-groups design, but note it is IN ADDITION to the usual mixed
design, where the treatment and control groups get repeated
testings. If possible, I would like to combine the advantages of
the wait-list design (for only 4 cases) with the independent-groups
design. I suspect this is impossible or would require mathematics
well beyond me -- that is, no standard analytic program could deal with
it. But just in case anybody knows of someone who, for example,
wrote a statistics dissertation on this issue, I'd like to hear about
it.
Secondly, I intend to argue that in effect, the 4 participants who were
in both the wait-list and treatment conditions can and should be treated
as 8 cases. That is, usually there would be concerns about counting
the same person as two subjects, but I contend that my solution is
actually conservative. You want to avoid counting a person twice in
the same treatment group because that artificially reduces within-group
variance, hence inflates relative between-group differences. In
fact, husband-and-wife pairs should not be assigned to the same condition
for that reason, unless you use a nested-group design (basically using
the couple as the unit of analysis rather than the individual). In this
case, the "matched" cases are assigned to "different"
conditions -- this would be similar to making sure that identical twins
went into different conditions.
Does anyone see any glaring holes in my logic? This will be peachy
for the client, but when the work is submitted for publication, I expect
some raised eyebrows. Does anyone know of a precedent or better
yet, an explicit argument for this in a previous publication? Or if
not, would any of the (relatively) big names on this list be willing to
be used as a reference?
Thanks!
Allan
Allan Lundy, PhD
Research Consulting
[hidden email]
Business & Cell (any time): 215-820-8100
Home (8am-10pm, 7 days/week): 215-885-5313
Address: 108 Cliff Terrace, Wyncote, PA 19095
Visit my Web site at
www.dissertationconsulting.net
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