In this article, the authors consider the possibility that groups could be different, because of the different conditions of a factor. This is as far as the analysis can extend: the consideration is restricted to groups characterized by the different category of the factor being considered. In many biological experiments, the factor considered may not be just a simple category, but it may be expressed in terms of order, or even as a continuous variable. If this can be done, then other helpful and powerful methods of statistical analysis can be used. The authors discuss linear regression analysis which considers variation in the "dependent" variable. (Contains 4 figures.)