Why is it so difficult to provide statistical data about the number of Quaternary jobs?

There is no difficulty in compiling employment statistics for firms in the quaternary sector. However, providing data on the number of quaternary jobs is made more difficult by the fact that some of those employed will be in firms whose main purpose is something else. For example, personnel employed in R&D in a pharmaceutical company may well be classified to the pharmaceutical sector. It would be necessary to collect information from sub-units in firms - for example, from pay-points.