The test-retest reliability and stability of the WAIS-R in a sample of mentally retarded adults.

Mazur et al. (1992) gave the WAIS-R IQ test twice to 50 adults with intellectual disability. The second test happened about 2.5 years after the first. The team wanted to see if the scores stayed the same without any special teaching in between.

Full-scale IQs barely budged. The average change was less than one point. Most people landed within five points of their first score. The test looked stable enough to trust small gains or drops as real change.

Whitaker (2008) pooled many studies and found the same thing: IQs usually hold steady. Yet that meta-analysis also warned that one in seven low-IQ adults shifts 10 or more points over three years. The 1992 data sit inside those averages, so both papers agree.

Dudley et al. (2019) moved the question forward. They showed the newer WAIS-IV still works in adults with ID, and they paired it with daily-living scores. Their low correlations remind us that IQ and adaptive behavior are separate tracks.

Prasher et al. (1995) ran a similar retest study but used the Reiss Screen for behavior problems. Good test-retest numbers again show that careful tools give steady readings in this population.

You can trust the WAIS-R (or its newer cousin) to flag real change. If a client jumps 12 points, dig into why—new meds, better hearing aids, or real learning. If scores barely move, stop reassessing every year just to check. Save those hours for teaching instead.