Assessment and diagnosis of mental illness in persons with mental retardation. Methods and measures.

Singh et al. (1991) drew a road map for telling true mental illness from everyday behavior problems in clients with intellectual disability.

They said: collect life history, interview caregivers, run rating scales, watch the client live, and do quick experiments. Only combine all five can you decide if the client needs psychiatry or just better behavior support.

The paper is purely theoretical. It lists no new data. Instead it argues that single tools always misclassify people with ID, so you must layer methods.

Later work both sharpened and challenged the 1991 plan. van Timmeren et al. (2016) used factor analysis on large clinical files and showed that dimensional symptom scores predict outcomes better than the yes/no categories N et al. assumed. In short, the field moved from "categorical diagnosis" to "dimension first."

Goodwin et al. (2012) cataloged dozens of validated scales that simply did not exist in 1991. Their review effectively updates the battery N et al. could only imagine.

Yet van Timmeren et al. (2016) systematic review warns that many new factor studies still use weak stats. So the 1991 caution remains valid: poor tools give poor answers, even when you call them "dimensional."

Keep the 1991 spirit: never trust one score. Start with a brief functional analysis, add a normed rating scale, and record direct observation. Then check if newer dimensional tools like those in van Timmeren et al. (2016) fit your client. This hybrid guards both against over-diagnosing depression and under-diagnosing real mood disorders in people with ID.