The great David Wechsler (author of the Wechsler Intelligence Scales) created the Wechsler Deterioration Index (WDI). This index was calculated by subtracting the sum of your scaled scores (calculated with reference to the peak age group: 20-34) on all the subtests that decline as a function of old age, from those subtests that hold up relatively well with old age, and then dividing by the sum of hold subtests.
WDI = (sum of hold tests – sum of don’t hold tests)/sum of hold tests
So if the sum of your hold subtests was 40, and the sum of your don’t hold subtests was 20, your deterioration index was said to be an astonishing 50%! The implication was that you lost half your intelligence.
This was a bit silly because the subtest scaled scores were at best an interval scale, not a ratio scale with a true zero point, so the use of percentages was misleading, but the concept was all the rage with clinicians in the middle of the 20th century.
The hold subtests were Vocabulary (word knowledge), Information (General Knowledge), Object Assembly (spatial relations), and Picture Completion (visual alertness). The don’t hold subtests were Digit Span (working memory), Similarities (verbal abstraction), Digit-Symbol (rapid eye-hand coordination) and Block Design (spatial analysis). Wechsler was careful to make sure both the hold and don’t hold subtests both included an equal number of verbal and performance (visual-motor) subtests because generally speaking verbal subtests hold up better with age so if not for this counter-balancing, naturally verbally gifted people would show a spuriously high degree of deterioration.
The formula was wildly popular because people who you’d expect to show a high degree of dementia such as alcoholics, schizophrenics and the elderly, all had huge WDIs.
Unfortunately, Wechsler died before James Flynn managed to popularize what would come to be known as the Flynn effect. The fact that performance on IQ tests had been rising over the 20th century.
Thus Wechsler probably went to his grave thinking that old people did bad on don’t hold tests because they had suffered mental deterioration, not realizing it may instead have been because their generation was always bad at those tests, even when they were young. In order to determine which subtests really do and don’t hold with age, you would need to do a longitudinal study of adults who took the WAIS both at their prime and in old age, or at the very least, you would need to adjust age declines in scores for the Flynn effect, which some scholars have tried to do.
Nonetheless in 1992, the original WDI made a comeback when it came to diagnosing ADHD when it was noticed that these tend to have high WDIs.