Measurement

Measurement is the assignment of numbers, according to rules, to physical or mental objects, attributes, traits, constructs, or concepts. The purpose of these [Page 642]numerical assignments is to enable comparisons, assessments, judgments, and evaluations through various mathematical computations and manipulations. These numbers may be obtained via the use of measurement tools or self-reports, or may be based on direct observations or judgments of overt phenomena or behaviors by human observers. Because of the convenience of operations of numbers and the general consensus regarding standard mathematical and statistical rules for comparisons and computations of numbers, measurement is considered a fundamental part of science. Galileo stated that, for the sake of knowledge, we must measure what is measurable and make measurable what cannot be measured.

The measurement of observable attributes of physical objects or phenomena is relatively direct. Human beings have engaged in these physical measurement activities for at least 5,000 years. The earliest known measurement of physical attributes was the use of the Royal Egyptian Cubit, which was defined as the length of the forearm from the elbow to the tip of the outstretched middle finger and is equal to a little less than 21 of today's inches, to measure the physical attribute of length. It is known that the Khufu Pyramid, which was completed around 2750 B.C.E., was constructed based on the use of the Royal Egyptian Cubit. As well, a number of lunar calendars were developed by various ancient civilizations to measure (i.e., to assign numerical values to) the concept of “passage of time.”

The measurement of educational and psychological attributes, including covert traits and overt behaviors, however, is a relatively recent phenomenon. One of the earliest rudimentary attempts to assign quantitative descriptors to educational and psychological attributes can be found in the civil service examination system of ancient imperial China. The system, called the Keju system, lasted from 606 to 1905. It had been an unofficial practice during approximately the past seven centuries of the system that, when an exam grader encountered an excellent sentence or choice of words in an exam essay, the grader would mark it with one or more circles or dots. Hence, an excellent essay would have a high density of circles and dots and a poor essay would have few to none. This practice did not have the numerical precision of modern educational/psychological measurement, but it was an early method in the use of numerical quantity (i.e., number or visual density of circles and dots) to represent an educational/psychological quality or attribute (i.e., excellent writing ability).

Precise methods of measurement of educational/psychological attributes did not develop systematically until the late 19th century. Gustav Fechner developed the first systematic method of modern psychological measurement when he attempted to measure the intensity of human sensations in 1860. He accomplished this by assigning a numerical degree of intensity based on the concept of “just noticeable difference.” The score for a sensation would move up one point on the scale for every “just noticeable difference” in the intensity of a particular sensation, such as pressure. In the 1880s and 1890s, there was a great deal of consensus among the early founding fathers of the then-new discipline of psychology, notably Sir Francis Galton, Charles Spearman, and James Cattell, that measurement was to be a critical characteristic of psychology if this new discipline was to attain the status of a science. These theorists focused primarily on the use of existing measures of physical attributes to gauge psychological attributes such as a person's reaction time, the amount of pressure that induces pain, the amount of time it takes a person to name a color, or Fechner's “just noticeable difference” in sensation—all in a general area of study called psychophysics or anthropometrics. In contrast, Alfred Binet developed the numerical concept of mental age as a measure of the covert psychological construct of intelligence in 1905, when he developed the Binet-Simon Scale of Intelligence. Lewis Terman standardized Binet-Simon's mental age in 1916 by dividing mental age by chronological age and multiplying the result by 100. The result is known as the Intelligence Quotient or Ratio IQ score. Subsequent to these early developments, a variety of new issues related to the choice of scoring metric, the precision or stability of the scores, and the meaningfulness of interpretations and uses of these scores arose; and many new developments took place in the next century. These may be divided into three major areas of concern: scaling methods, reliability theories, and validation methods.

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