Limitations in the use of job exposure matrices

A Job Exposure Matrix (JEM) is a cross classification of occupations, industries and exposures within a given job. A general JEM covers all occupations and industries. Severe misclassification by using such a JEM is hardly avoidable. Use of a general JEM will therefore rarely be able to identify risk ratios in the moderately elevated range. However, a general JEM could be an important tool in looking for differentiated misclassification, a frequently mentioned problem in case-control studies. Principles of this kind of use is given in the text.