In the absence of the detailed information that would be required for the foregoing type of analysis,another less precise method is resorted to. This is the standard method used by the Registrar-General in the "Decennial Supplements, Occupational Mortality Tables", where a yearly estimate of the employed population in the relevant occupations is broken down into age groups, and from this an estimate of the expected frequency of the event under consideration is made by the application of the agespecific
rates for that event determined for the general population. The determination of the observed number of events requires that the subject shall be in the specified environment at the time that the event takes place. Thus all the information that can
be utilized is that relating to workers who, once having entered the specified occupation, remain in that occupation until the event being studied overtakes
them. The increase of precision of the former method of analysis described above over this latter form will depend upon the labour stability of the industry concerned, but may easily be of the order of five-fold or more.