ABSTRACT
The overall length of a longitudinal study may be shortened somewhat by making it semi-longitudinal. Longitudinal studies done in ‘rounds’ can suffer in the same way but, with continuous monitoring the problem disappears. Another variant of the basic longitudinal study is exemplified by the British Labour Force Survey. In the typical case, a series of age cohorts is followed until the cohorts overlap. Interviews are conducted throughout the year at the rate of 5000 households per month. A similar sort of crackdown in the UK involved introduction of the breathalyser in (September) 1967. Whooping cough vaccine is known to be effective in preventing the illness in an individual. But it occasionally causes death or brain damage. It is therefore of some interest to ascertain whether the introduction of the vaccine in the UK round about 1957 was of benefit to the community as a whole.
