Assessment instruments that do measure traits or abilities are called normative instruments. The term refers to the normal distribution of the population. During the development of a normative instrument, a large sample of the population is given the assessment. This population is composed of various demographic segments that are representative of the population which will be subject of the assessment. The sample includes a reasonable distribution of ages, races, education levels, types of jobs, and any other appropriate factors. It is the distribution of the scores of this population that creates the normal bell curve or normal distribution that becomes the “measuring stick” of the assessment. This allows any individual to be compared to any other individual or to the elements of any job. It becomes a common form of measurement or a way to objectively describe specific behaviors that are part of a job.
Once the normal distribution has been established, it is divided into sections with each one representing a percentage of the population. The most common method of this divides the scale into standard tenths with each part representing one half of a standard deviation. This is called a sten scale. Early normative instruments used standard ninths, or stanine scales. This method cuts the normal curve into nine parts of equal width, each one representing one half of a standard deviation. This was when computers had limited fields of characters, so one digit was better than two digits. Some other normative instruments divide their scale into three, five or six sections. Any one of these methods is acceptable, and each offers advantages of different kinds.