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alleles involved. It is also very small, less than about 0.01 in the United States. There has not been the extensive sampling of subpopulations and geographical areas for PCR-based systems that has been done with VNTRs. New data show low values of image and good agreement with HW and LE. The uncertainty range appears to be about the same as that for VNTRs. We therefore believe that STRs can take their place along with VNTRs as forensic tools. They circumvent most of the matching and binning problems that VNTRs entail.

The Ceiling Principles

The most controversial recommendations of the 1992 NRC report are the ceiling principle and the interim ceiling principle. They were intended to place a lower limit on the size of the profile frequency by setting threshold values for allele frequencies used in calculations. The ceiling principle calls for sampling 100 persons from each of 15-20 genetically homogeneous populations spanning the racial and ethnic diversity of groups represented in the United States. For each allele, the highest frequency among the groups sampled, or 5%, whichever is larger, would be used. Then the product rule would be applied to those values to determine the profile frequency. But the data needed for applying this principle have not been gathered. We share the view of those who criticize it on practical and statistical grounds and who see no scientific justification for its use.

The 1992 report recommended further that until the ceiling principle could be put into effect, an interim ceiling principle be applied. In contrast to the ceiling principle, the interim ceiling principle has been widely used, and sometimes misused. The rule says: "In applying the multiplication [product] rule, the 95% upper confidence limit of the frequency of each allele should be calculated for separate US 'racial' groups and the highest of these values or 10% (whichever is larger) should be used. Data on at least three major 'races' (e.g., Caucasians, blacks, Hispanics, east Asians, and American Indians) should be analyzed."

The interim ceiling principle has the advantage that in any particular case it gives the same answer irrespective of the racial group. That is also a disadvantage, for it does not permit the use of well-established differences in frequencies among different races; the method is inflexible and cannot be adjusted to the circumstances of a particular case. The interim ceiling principle has been widely criticized for other reasons as well, and we summarize the criticisms in Chapter 5 (p 157). We agree with those criticisms.

Our view is that sufficient data have been gathered to establish that neither ceiling principle is needed. We have given alternative procedures, all of which are conservative but less arbitrary.

Although we recommend other procedures and believe that the interim ceiling principle is not needed, we recognize that it has been used and some will probably continue to use it. To anticipate this possibility, we offer several suggestions in



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