Journal of Dental Research, Vol 71, 1516-1521, Copyright © 1992 by International & American Associations for Dental Research Online Journals
Determination and selection of the optimum number of sites and patients for clinical studies
P. P. Hujoel and T. A. DeRouen
Department of Dental Public Health Sciences, School of Dentistry, University of Washington, Seattle 98195.
Calculating required sample sizes is a critical step in the design of any
study. For dental studies, the sample size needs to be specified at two
levels: (1) the number of patients (n) enrolled in the study, and (2) the
average number of sites (m) examined per patient. In general, m and n
should be selected in such a way that the precision of the research
findings is maximized, while the cost of the study is minimized. This
objective can be realized by taking stock of the components of variation
and the costs involved with enrolling patients and examining sites. The
research cost for n patients ($C1/patient), at an average of m sites per
patient ($C2/site), can usually be approximated by nC1 + nmC2. The
precision varies as a function of the variance components, m, and n. To
optimize precision for a fixed cost, the average number of sites examined
per patient (m(opt)) should be approximately equal to [formula: see text]
where rho is the within-patient correlation coefficient of the
site-specific variable measured. When m(opt) is approximately equal to or
in excess of the average number of sites available per patient, whole-mouth
examinations are indicated. When m(opt) is considerably smaller than the
average number of sites available, the sample of optimum size should be
obtained by some random mechanism. Examination of a number of sites
considerably different from m(opt) results in a waste of resources,
regardless of the number of patients studied. Standard statistical analyses
for determination of the patient sample size required to obtain a
pre-specified precision or power are discussed.
