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Research on one foot, two feet, or one person

Discussion in 'General Issues and Discussion Forum' started by Craig Payne, Jul 15, 2008.

  1. Craig Payne

    Craig Payne Moderator


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    I have recently reveiwed 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 guility 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 independance. 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):
    Hylton Menz brought this to our attention in the podiatric literature:
    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)?
  2. Have seen this problem myself in a number of manuscripts that I have reviewed recently too. What we used to do was to test left foot data against right foot data for statistical significance and then pool the data if it was not different.

    Playing Devil's advocate: As by their very nature left and right feet are opposite, are they really paired data? The argument that they have the same body weight, the same blood supply etc, doesn't really hold true either. How dissimilar does something have to be before it can be considered not paired? Taking this argument to completion, if the researchers first demonstrated statistical difference between the left and right data could they then pool the data? This would mean that we have a binomial population and the assumptions of normal distribution would be violated, but the good old t-test is pretty robust isn't it. :drinks
  3. Kenva

    Kenva Active Member

    Craig, Simon,

    Thanks for the above, this one was bugging me for quite some time and you could see that there was no consensus on the topic. I can imagine that it's going to take a while before the pooling of data duel is going to end...

    What the most important aspect of this discussion is (should be), is that podiatric research should take a stand and stick with it.
  4. Adrian Misseri

    Adrian Misseri Active Member

    Being that in the human body, both feet are not identical, why not consider each foot as a subject? true they are not 'strictly' independant, being that they are fromteh same person and therefore similar, but then again mosat of the 'RCT's do not really have a randomised population, it's usually a convenience sample, or a clinically controlled trial, or whoeve happens to walk in. Are we clutching at straws??
  5. Craig Payne

    Craig Payne Moderator

    Other disciplines have made that decision. You would not get away with using two eyes in the opthalmological literature; you won't get away with using two knees in the rheumatological literature; you won't get away with it in the biomechanics literature; why is that you can still get away with it in the podiatric literature? Why should podiatric research be at a 'lower' level than other disciplines?
    Last edited: Jul 16, 2008
  6. markjohconley

    markjohconley Well-Known Member

    great little thread here, I was ignorant of the issue. Just querying the use of 'lower'. Why not 'different'?
  7. Admin2

    Admin2 Administrator Staff Member

  8. Adrian Misseri

    Adrian Misseri Active Member

    So in doing research, do we need to specify if it's left or right foot? Or can we just ramdomly pick a foot?
  9. Craig Payne

    Craig Payne Moderator

    Fom all my understanding, it does not matter. You can use just the left or right foot; randomly select one foot; use the symptomatic foot; use the most symptomatic foot.

    A while ago one of my Hons students did an intensive search to see what was more appropriate - ie should he use the left in all subjects or the right in all subjects; or randomly select the left or right. He could not find any convincing arguments for either approach (which is what I told him in the first place :bang: )

    Incidently, there are situations in which data from both feet is needed (eg if you are doing a study comnparing the left side to the right side, but the statistical prerequisites are not breached by that kind of analysis)
  10. First I would want the researcher to demonstrate that there is no statistical difference between the left and right foot data for the variables under investigation. If no differences were identified, then it would be ok to randomly select right or left by the flip of a coin. This could be for the population or subject by subject. But since they would need to show no difference in the first place they might as well go on to examine both right and left foot data independently! And if they do find differences.... is it really paired data?????? See my previous post.

    Think about this: lets say we measured HA angle in twenty subjects (40 feet) and the mean HA angle in the left foot was 5 degrees (3.5, 6.5) and in the right foot it was 15 degrees (12, 17) would it be safe to randomly select one foot for analysis over the other? If I choose to analyze the right foot data only what bias would this introduce? Adrian's point re: selection is well made
    Last edited: Jul 16, 2008
  11. Hylton Menz

    Hylton Menz Guest

    Dear all,

    Craig has asked me to add my two cents worth to the discussion on this issue, which was initially brought to my attention by Lloyd Reed at QUT. My view on this is explained in two papers:

    Menz HB. Two feet, or one person? Problems associated with statistical analysis of paired data in foot and ankle medicine. Foot 2004;14:2-5.

    Menz HB. Analysis of paired data in physical therapy research: time to stop double-dipping? J Orthop Sports Phys Ther 2005;35:477-478.

    There's fair degree of overlap between these two papers, as the second one was an invited editorial in which I was specifically requested to briefly summarise the key points of The Foot paper.

    I've uploaded the PDF of The Foot paper (and a reply by Dr Tony Redmond) for those interested in exploring this issue in more detail.

    As a general rule, pooling data should be avoided for the reasons outlined in the paper. However, in some cases it may be justified, particularly in situations where the variables being examined are very foot-specific, there is a high degree of variation between feet, and the condition being examined is frequently unilateral.

    I guess the bottom line is that authors who decide to pool data from both feet should provide a conceptual and statistical justification for doing so, rather than just double-dipping to increase sample size (which I suspect is a very common reason for doing it).

    Attached Files:

  12. Craig Payne

    Craig Payne Moderator

    Just at ESM mtg in Dundee and in one of the papers from todays session the reseachers used another option: they used the dominant limb (by getting the participant to nominate which limb they preferred to stand on in a one legged stance position). I guess this option is fine unless the research question's answer is affected by limb dominance issue.
  13. NewsBot

    NewsBot The Admin that posts the news.

    Statistical Consideration for Bilateral Cases in Orthopaedic Research
    Moon Seok Park, Sung Ju Kim, Chin Youb Chung, In Ho Choi, Sang Hyeong Lee, and Kyoung Min Lee
    J Bone Joint Surg Am. 2010;92:1732-1737
  14. Jeremy Long

    Jeremy Long Active Member

    If I may take this space to voice my own personal vendetta among the majority of orthoses research:

    Far too many studies are donw with the subjects wearing their own, usually-way-too-worn shoes. How in the world would anyone achieve reasonable levels of control under this kind of situation?
  15. Craig Payne

    Craig Payne Moderator

    Its actually quite easy to control for this.

    The whole point of using randomization is that you end up with 2 groups that are theoretically identical in all their characteristics (ie age, weight, foot type, etc). Each group theoretically will have equal proportion of good and bad shoes in them. Many RCT's to stats tests on a whole range of baseline characteristics to show the groups are teh same.

    As you have two "identical" groups, you then apply an intervention to one group and follow them over time - this means the groups are the same except for one characteristic (the intervention) ---- you can then determine what the effects of that intervention was.

    If you also intervened to change the footwear as well, then what actually worked? Was it the footwear change or was it the orthotic change?

    UNLESS, your research question was actually about what are the combined effects of footwear and orthotic change, in which case, you do change the footwear. If your research question was just looking at orthotic's then you do not change the shoes.

    Pragmatically, we have to work clinically with people who do not change their shoes, so the RCT's are just reflecting that.
  16. NewsBot

    NewsBot The Admin that posts the news.

    Bilateral Clubfeet Are Highly Correlated: A Cautionary Tale for Researchers.
    Gray K, Gibbons P, Little D, Burns J.
    Clin Orthop Relat Res. 2014 Jul 15.
  17. NewsBot

    NewsBot The Admin that posts the news.

    Analysis of data collected from right and left limbs: accounting for dependence and improving statistical efficiency in musculoskeletal research
    Sarah Stewart et al
    Gait and Posture; Article in Press

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