Wednesday, July 16, 2008

Research on one foot, two feet, or one person

I have recently reviewed several manuscripts that I recommended that editors not publish due to a fundamental flaw in the methodology. It concerned me enough to post a thread here about it (and will freely admit that I have been guilty of this in the past, but times change as we learn more).

One potentially appealing thing about doing foot or podiatry research is that each subject has two feet, meaning that if you use both feet in the data, you have either doubled your sample size or halved the number of subjects used.

HOWEVER, a key assumption of almost all statistical tests is that the subjects in the sample are independent of each other ..... this means that you can not use two feet from the same person in the sample as they are related (not independent of each other; they are paired) - they have the same body weight; the same blood supply; etc etc ...

The use of the two feet of one subject is no longer acceptable in research due to this lack of independence. This is a common issue in the opthalmologic literature (two eyes or one eye?); the orthopaedic literature (two limbs or one?); the rheumatological literature (eg one knee or two):
"SUTTON et al. Two knees or one person: data analysis strategies for paired joints or organs Ann Rheum Dis.1997; 56: 401-402"

Hylton Menz brought this to our attention in the podiatric literature:
"H . Menz: Two feet, or one person? Problems associated with statistical analysis of paired data in foot and ankle medicine . The Foot , Volume 14 , Issue 1 , Pages 2 - 5, 2004"

Why are researchers still using both feet; still submitting the data for publication using both feet in the analysis; and why are journal editors still permitting them to be published (esp in podiatric journals)?

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