Clear Writing: Ten Principles of Clear Statement
http://muextension.missouri.edu/xplor/comm/cm0201.htm

University of Missouri
1973

Abstract:

If you want to test the clearness of your writing, you may wish to consider using a 'fog index.' Fog indexes measure the complexity of writing samples, and often provide a means of calculating the reading or educational level required to understand a particular passage. Some fog indexes are available as computer software programs, or you may do the calculations yourself.