The data in Table gives the frequencies of anencephalus, spina bifida and other malformations of the central nervous system among live births in various South Wales communities. The principal object of the study was to investigate the effect, if any, of water hardness on the incidence of such diseases.

(a) Find a good model to study the possible effects of water hardness and differences between manual and non-manual workers on the incidence of CNS disorders (hint: you need to create a variable which is the rate of CNS incidence).