Dear Susan,
Greetings from coastal Suffolk, UK.
If I remember correct this question or a related issue has been raised and
addressed before on this list. But it is not unusual for questions to be
repeated at intervals. The previous posting was about the level of
randomising in a detal trial; Should one randomise patients/ Teeth / Roots /
Canals? Question posted on 23rd Feb 2001 and an answer on 1st of March 2001.
You can access them under archives at
http://www.jiscmail.ac.uk/lists/EVIDENCE-BASED-HEALTH
As Dentistry is a field where one makes multiple measurements on the same
patient. Hence I thought it might be interesting to see what the guidlines
for authors of the BDJ say. Please find below the section on Unit of
analysis from these guidelines.
"Units of analysis
In dental research it is common for several measurements to be made on the
same patient (e.g. measurements may be taken from several sites in the same
mouth, or from the same site on several different occasions), but the focus
of interest usually remains the patient. Measurements taken from different
sites within the same mouth should naturally be expected to be more similar
to each other on average than measurements taken from different mouths.
Failing to consider this situation of 'correlated' measurements causes
multiple counting of individual patients and can lead to seriously distorted
results. In particular it inflates the sample size and can lead to spurious
statistical significance. Since the patient is the unit of the
investigation, the patient should also be the unit of analysis.
If the patient is not treated as the unit of analysis, complex specialised
statistical methods need to be employed (e.g. Multilevel/hierarchical
modelling)". This level of stats is beyond me but I am sure some of the
Stats Gurus (I can visualise a few) on the list will throw more practical
light on this issue. The BDJ guidlines are available on line @
http://www.nature.com/bdj/about/submission_statistics.html
It refers to a book by Altman G which you might find useful.
Hope this is of some interest/help.
Cheers & regards,
Badri
Dr.P.Badrinath M.D,M.Phil,(Epid)PhD(Cantab)DFPHM),MPH(Distinc)
Specialist Registrar in PHM & Honorary Clinical Lecturer,
Suffolk Public Health Network & University of Cambridge,
PO Box 170, St.Clement's Hospital,
Ipswich IP1 4LA, UK.
http://myprofile.cos.com/badrishanthi
_________________________________________________________________
Send and receive Hotmail on your mobile device: http://mobile.msn.com
|