1. Biostatistics Unit; Group Health Research Institute; Seattle WA 98101 U.S.A.
2. Department of Biostatistics; University of Washington; Seattle WA 98195 U.S.A.
3. Department of Statistics; North Caroline State University; Raleigh NC 27695 U.S.A.
4. NYS Psychiatric Institute; Columbia University; New York NY 10032 U.S.A.
5. School of Computer Science; McGill University; Montreal, Quebec H3A 0E9 Canada
6. Department of Statistics; University of Michigan; Ann Arbor MI 48109 U.S.A.