The mental age was divided by the number of months of the child's chronological age and multiplied by 100 to get a whole number. This then was a criterion based test. In other words the child's performance was judged based upon his or her performance as compared to certain established criterion. The deviation IQ compared the subject's performance to a normed and standardized group in which the subject fell. Thus the IQ was more accurate because it was statistically more sound, gave information as to how the subject compared to others in the same age group, and gave statistical information.
Most intelligence tests have many things in common.
They generally start with easier items and progress to more difficult items. They measure more than one area of intelligence. There is an attempt to make them interesting so that the subject puts forth his or her best effort. They are normed and standardized on a sample population. There are different theories of intelligence.