  It is often important to incorporating covariate information in the design of
clinical trials. In literature, there are many designs of using stratification
and covariate-adaptive randomization to balance on certain known covariate.
Recently Zhang, Hu, Cheung and Chan (2007) have proposed a family of
covariate-adjusted response-adaptive (CARA) designs and studied their
asymptotic properties. However, these CARA designs often have high
variabilities. In this paper, we propose a new family of covariate-adjusted
response-adaptive (CARA) designs. We show that the new designs have smaller
variabilities and therefore more efficient.
