The ICAP’s popularity is largely due to its usefulness and accuracy, derived from quality standardization and norming.
Standardization is the process of developing a test that reliably and validly measures or discarding those that don't work, adding items where there are gaps in difficulty, and reanalysis. It also involves developing standard testing procedures and materials. The result should be a test that reliably measures the same thing the same way each time, so that scores are comparable. A standardized test should also demonstrate validity, meaning that it actually measures what it portends to measure. A test score should correlate with something in the real world.
Norming is the process of determining average scores for average people. Many tests are standardized, but few are normed because norming is an expensive process which usually means testing hundreds or thousands of carefully selected individuals -- school children throughout the U.S., for example. Norms are important because they help give test scores meaning - something to compare to. A raw score of 7, for example doesn't mean much. It means more to know that a score is at the 50th percentile for an individual of a certain age.
In norming, a large number of individuals are given a standardized test in order to determine average (normal) test scores, often averages for each age or other sub-group. This permits test scores to be compared to average, to each other on a relative basis, or to scores on other normed tests. The three major types of norm scores are age scores, percentiles, and standard scores.
Tests can be normed on more narrowly defined populations, although the usefulness of these scores varies with the definition of the norm group. "Developmentally disabled adults," for example, can range from profoundly mentally retarded to almost average. Norms on such a loosely defined group are equally loosely defined. Therefore the following comparisons differentiate norms for average non-handicapped individuals from data on supplemental standardization groups of people with handicaps.