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Randomization of small patient groups

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  • Randomization of small patient groups

    Dear readers,

    I am facing a problem which I am sure many people planning clinical
    studies with gait analysis will have had before:

    In a study monitoring surgical outcomes in a prospective randomized
    trial of two groups the number of patients to be recruited in a
    reasonable time typically is very limited (between 10 and 50). We are
    thinking about a study comparing two groups with about 25 patients in
    each group monitoring the gait pattern pre and post surgery.

    Ideally one would have the groups starting at the same level of walking
    ability and with comparable walking patterns. A simple randomization
    scheme for this small number however, might by accident lead to
    significant differences between the groups pre-op.

    In the literature I found a method called "minimization" where strata
    are defined with respect to relevant influencing factors as eg. the
    walking ability pre-op. The probability for allocating a patient to a
    certain group then could be driven by the individual patients' walking
    ability. In small sample numbers however this could effect in a hidden
    pre-determination for group allocation for the majority of the samples
    (patients). Talking to a biometrics I learned that in this case such a
    procedure is not acceptable.

    However, nobody could explain to me why such a procedure is not correct
    (even in the case that all patients might be pre-determined for
    allocation). As long as this procedure guarantees that neighter the
    surgeon nor the staff taking the gait analysis data nor any person
    involved in the recuitment process has the possibility to interfere or
    influence the allocation scheme I would think that it is a reasonable
    procedure for group allocation.

    Further, taking that this method is acceptable, I do not know which
    feature to choose for stratification regarding walking ability and/or
    walking pattern. One very simple parameter would be body height
    normalized walking speed. Another possibility would be the Gillette Gait
    Index (Schutte et al. Gait Posture 2000). Any other suggestions?

    Any comment is welcome.



    Dr. Sebastian Wolf

    Gaitlab Manager

    Department of Orthopedic Surgery

    University of Heidelberg

    Schlierbacher Landstr. 200a

    D-69118 Heidelberg - GERMANY

    phone: +49 6221-96 6724

    fax: +49 6221-96 6725