It depends which test is used and there are limits in magnitude to practice effects, i.e. you can't coach anyone to a perfect score in a test with a large bank of test questions. Psychometricians are aware of these critiques. Tests like the SAT, GRE, GMAT etc., lightly disguised IQ tests all, are useful for predicting academic performance among other things.
>Effects of practice on the Wechsler Adult Intelligence Scale-IV across 3- and 6-month intervals.
Estevis E1, Basso MR, Combs D.
A total of 54 participants (age M = 20.9; education M = 14.9; initial Full Scale IQ M = 111.6) were administered the Wechsler Adult Intelligence Scale-Fourth Edition (WAIS-IV) at baseline and again either 3 or 6 months later. Scores on the Full Scale IQ, Verbal Comprehension, Working Memory, Perceptual Reasoning, Processing Speed, and General Ability Indices improved approximately 7, 5, 4, 5, 9, and 6 points, respectively, and increases were similar regardless of whether the re-examination occurred over 3- or 6-month intervals.
> Practice Effects for the Stanford–Binet Intelligence Scales
The Stanford-Binet Intelligence Scales—Fifth Edition (SB5) is a widely used
assessment tool for measuring intelligence (Roid, 2003). According to Roid, a key
advantage of this intelligence test’s most recent revision is that it includes improved lowend
items for better measurement of young children or adults having mental retardation.
Sbordone, Saul, and Purisch (2007) report that the range of the SB5 was expanded to
allow the assessment of very low and very high levels of cognitive ability. Roid and
Barram (2004) indicate that the practice effects on the SB5 were smaller than expected.
For example, the nonverbal IQ of the SB5 showed shifts of only 2 to 5 points as
compared to the 4 to 13 points on the Performance IQ of the Wechsler scales (i.e., the
WAIS-III and WISC-III). Roid and Barram add that the lower shift, and thus practice
effect, is even more notable given that the retest period for the SB5 was 5 to 8 days
versus 23 to 35 days on average for the Wechsler scales.
>Effects of practice on the Wechsler Adult Intelligence Scale-IV across 3- and 6-month intervals. Estevis E1, Basso MR, Combs D.
A total of 54 participants (age M = 20.9; education M = 14.9; initial Full Scale IQ M = 111.6) were administered the Wechsler Adult Intelligence Scale-Fourth Edition (WAIS-IV) at baseline and again either 3 or 6 months later. Scores on the Full Scale IQ, Verbal Comprehension, Working Memory, Perceptual Reasoning, Processing Speed, and General Ability Indices improved approximately 7, 5, 4, 5, 9, and 6 points, respectively, and increases were similar regardless of whether the re-examination occurred over 3- or 6-month intervals.
> Practice Effects for the Stanford–Binet Intelligence Scales The Stanford-Binet Intelligence Scales—Fifth Edition (SB5) is a widely used assessment tool for measuring intelligence (Roid, 2003). According to Roid, a key advantage of this intelligence test’s most recent revision is that it includes improved lowend items for better measurement of young children or adults having mental retardation. Sbordone, Saul, and Purisch (2007) report that the range of the SB5 was expanded to allow the assessment of very low and very high levels of cognitive ability. Roid and Barram (2004) indicate that the practice effects on the SB5 were smaller than expected. For example, the nonverbal IQ of the SB5 showed shifts of only 2 to 5 points as compared to the 4 to 13 points on the Performance IQ of the Wechsler scales (i.e., the WAIS-III and WISC-III). Roid and Barram add that the lower shift, and thus practice effect, is even more notable given that the retest period for the SB5 was 5 to 8 days versus 23 to 35 days on average for the Wechsler scales.