Scaling possibly --- the odds ratio may give you the relative changes in
odds from increasing the concentration by one unit on the scale in which
the concentration is measured. Since the range of application seems to be
about 10, 260 units more meaningful results might be quoted for units
of say 100 increase where odds ratio would be about 1,47.
--- or at least that is my reading of what you say from information given
-- relative odds for increases of 1000 units would be as high as 59.
AF
At 17:17 16/09/2004, you wrote:
>Dear Allstaters,
>I am a little bit puzzled about something and I was wondering if someone
>had any suggestion related to this…
>I have the concentration results of an assay which clinicians would like
>to use in order to discriminate between subjects of CAD (Coronary Arterial
>Disease) and subjects of no CAD.
>The CAD group has 109 subjects and the no-CAD 53.
>I try to perform logistic regression modelling for the probability of a
>subject having CAD. However, the results are a little bit confusing.
>Although, the model fits the data well and the p-value of the estimate of
>the assay was highly significant (<0.001), the odds ratio of it came out
>to be 1.004(1.002, 1.006)!!!
>Therefore, I got confused.
>Then, I implemented a ROC plot which gave me really nice plot of AUC of
>0.895 (the two groups were found to have significantly different means
>with the CAD group having bigger mean concentration than the no-CAD).
>Then I thought that maybe it was the different group sizes (53 non-CAD
>versus 109 CAD). I generated some data so that the group’s size would be
>quite similar (106 non-Cad versus 109 CAD this time.) Again, the estimate
>of the assay was found to be significant (P<0.001) but the odds ratio
>1.004(1.003, 1.005).
>I am thinking that these strange results could be due to the very high
>range of concentrations I have to deal with, (47µg/ml-10307.40µg/ml).
>Anybody has any suggestion..?
>Ioanna
>
>_________________________________________________________________
>It's fast, it's easy and it's free. Get MSN Messenger today!
>http://www.msn.co.uk/messenger
Antony Fielding
Reader in Social and Educational Statistics
Department of Economics
University of Birmingham ,
United Kingdom
& Visiting Research Fellow,
Centre for Multilevel Modelling
Institute of Education, University of London,
United Kingdom
& Team member and consultant
National Evaluation of the Children's Fund
University of Birmingham
Address: Department of Economics, University of Birmingham, Birmingham B15
2TT , UK
Tele: 0121-414-6649 ( direct)
0121 -415- 8671 ( NE-CF direct)
0121-414-6640 ( dept office)
0121-414-7377 ( fax)
0121 -449-7986 or 0121 -246-2932 ( home)
07710-196121( mobile)
email : [log in to unmask]
and [log in to unmask]
(note carefully spelling of my christian name-- along with Lord Snowdon and
a few others I have no 'h' in it)
web page : www.economics.bham.ac.uk/people/fieldingt.htm
