Hi Cagri,
Without knowing your groups, and not having read your manuscript, I'm not
going to attempt to interpret your reviewer's comments. If it is not
appropriate to assume a linear relationship between your diagnostic
groups, then you should consider politely making that case to the
reviewer, and perhaps conduct relevant pairwise group comparisons and/or
the conjunction analyses that I suggested earlier.
Good luck,
-MH
> I see, thank you very much anyways.
> I am wondering if it's my misinterpretation of the reviewers comment, so I
> am copying the exact words of the reviewer to give a better idea about
> what is asked from us (I just changed the group names):
>
> " the examination of a potential continuum of abnormalities between these
> groups is likely the more adequate approach. Given the limited statistical
> power of this study this is probably best examined in the context of a
> multiple regression model with a group variable (e.g., Group X = 4, Group
> Y = 3, Group Z = 2, healthy volunteers = 1) and an independent estimation
> of (potentially unequal) group variances. Given the strong a priori
> evidence for a continuous increase in gray matter deficits in prefrontal
> and temporal cortices over groups, one-sided testing of such a model
> appears legitimate."
>
> Many thanks
>
> Cagri
>
>
> On Fri, 6 Jan 2012 08:07:23 -0600, Michael Harms <[log in to unmask]>
> wrote:
>
>>I've never had to test for an "ordered continuum" between groups, so
>>maybe others will chime in. Perhaps you could do the conjunction of the
>>regions that satisfy 1 > 2, 2 > 3, and 3 > 4 ?
>>
>>good luck,
>>-MH
>>
>>On Fri, 2012-01-06 at 12:34 +0000, Cagri Yuksel wrote:
>>> Thank you Michael, that was enlightening. The answer is no, we can not
>>> assume that there is a linear relationship between these diagnostic
>>> groups.
>>>
>>> So how should a model be testing a continuum of GM abnormalities
>>> between these 4 diagnostic groups using a multiple regression model ? I
>>> really can not think of anything at this point.
>>>
>>> Cheers
>>>
>>> Cagri
>>>
>>> On Thu, 5 Jan 2012 10:56:47 -0600, Michael Harms
>>> <[log in to unmask]> wrote:
>>>
>>> >Whether or not a linear model relating the groups makes sense depends
>>> on
>>> >on the specific groups, so I don't know whether it makes sense in your
>>> >context or not. I'll just note that modeling a linear relationship
>>> >between groups is a specific hypothesis that assumes that each step up
>>> >in the "group" variable yields an identical change in the dependent
>>> >variable (since all the groups were themselves spaced by a delta of 1
>>> >unit). This is *not* the same as hypothesizing that there is merely a
>>> >continuum in the DV such that 1 > 2 > 3 > 4 (or 1 < 2 < 3 < 4).
>>> >
>>> >cheers,
>>> >-MH
>>> >
>>> >On Thu, 2012-01-05 at 16:38 +0000, Cagri Yuksel wrote:
>>> >> Hello Michael,
>>> >>
>>> >> Thank you for your answer. Yes, I realized my mistake about the
>>> interpretation of the results right after I sent the message.
>>> >>
>>> >> These diagnostic groups are related and this analysis is to test an
>>> a priori hypothesis about a possible continuum of GM abnormalities
>>> in these groups, that's why I was thinking a linear model.
>>> >>
>>> >> Do you think it makes sense ? Do you have other suggestions?
>>> >>
>>> >> Thank you again,
>>> >>
>>> >> Cagri
>
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