Hi
James M. Clark
Professor of Psychology
204-786-9757
204-774-4134 Fax
[EMAIL PROTECTED]
>>> <[EMAIL PROTECTED]> 13-Nov-07 7:15:03 PM >>>
On 12 Nov 2007 at 13:03, Ken Steele wrote:
But there is something that's curious about these groups. For the
reported comparisons, there were actually two different controls. One was
an "attention-control" ("no treatment sensitization/contact control"; n =
123); the other was just a "control" ("no-treatment, no-contact control";
n = 58). They didn't give details, but I assume the attention-control was
a kind of minimal placebo group. But after going to the trouble of
creating two kinds of control group, they didn't report any comparisons
using them, except for comparing one with the other. Finding no
difference, they used that as justification for combining them. All
comparisons with the intervention group were made using the combined
group.
That's a problem because their control group then is a strange hybrid,
neither purely no-treatment nor attention-placebo. I suspect they did it
because they weren't getting much joy from their comparisons with each
control group separately. But it does limit whart they can conclude about
their intervention (not that they felt so constrained, of course).
JC:
Do they give the means for the two control groups separately? If not (or if
the means are presented and show a certain pattern), the collapsing could mask
a problem. The following SPSS program illustrates that it is possible to have
no significant difference among your 3 means, to have no difference between
your treatment (g3 = 3 in the program) and the placebo group (g3 = 2), and yet
to produce a significant difference between your treatment and the aggregate of
the two control groups. So what they did could be used to mask a lack of any
difference between the treatment and placebo groups, somewhat problematic for
the interpretation of the "treatment" effect.
set seed = 123459832.
input program.
loop s = 1 to 180.
end case.
end loop.
end file.
end input program.
compute g3 = trunc((s-1)/60)+1.
compute g2 = 2.
if s < 61 g2 = 1.
comp dv = rv.norm(50, 10) + (g3-2)*3.6.
manova dv by g3(1 3) /print = cell
/contr(g3) = SPEC(1 1 1 0 -1 1 -2 1 1).
manova dv by g2(1 2) /print = cell.
Take care
Jim
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