Mental health measurement has been based primarily on subjective judgment and classical test theory. Impairment is determined by a total score, requiring that all respondents be administered the same items. An alternative is adaptive testing in which different
individuals may receive different scale items that are targeted to their specific impairment level. We have developed adaptive depression, anxiety, mania and suicidality tests based on multidimensional item response theory. The shift in paradigm is from small
fixed length tests with questionable psychometric properties to large item banks from which an optimal small subset of items is adaptively drawn for each individual, targeted to their level of impairment. Results to date reveal remarkable increases in precision
of measurement and dramatic decreases in patient burden. For example, depressive severity can be measured using an average of only 12 items in 2 minutes anywhere on the planet from a bank of 400 items, yet maintains a correlation of r=0.95 with the 400 item
scores. Applications to psychiatric epidemiology, genetics, global health, large-scale screening, ecological momentary assessment and assessment across the lifespan are discussed.