Comparison of methods for estimating premorbid intelligence

Figures & data

Figure 1. Number of academic publications in which NART-R (solid line), WTAR (dashed line) and Advanced Clinical Solutions/Test of Premorbid Functioning (ACS/TOPF) (dotted line) neuropsychological tests were cited for each year from 2011 to October 2017. Google Scholar (5 October 5 2017) citation counts based on [Nelson and Willison (1991). National Adult Reading Test (NART). NFER-Nelson] for NART-R; [Wechsler (2001). Wechsler Test of Adult Reading: WTAR. Psychological Corporation] for WTAR, and combined counts from [Pearson (2009). Advanced Clinical Solutions for WAIS-IV and WMS-IV: Administration and scoring manual. The Psychological Corporation, San Antonio] and [Wechsler (2011). Test of Premorbid Functioning. UK Version (TOPF UK). UK: Pearson Corporation] for ACS/TOPF.

Table 1. WAIS-IV performance and demographics.

	Mean	SD	N	Sample proportion (%)
Age	40 . 00	16 . 78	92	100
FSIQ	108 . 52	12 . 71	92	100
80–90			7	8
91–100			15	16
101–110			30	33
111–120			23	25
121+			17	18
Social class^a	3 . 15	1 . 41	78	85
I Professional			8	10
II Managerial/technical			15	19
III Skilled non-manual			26	33
IV Partly skilled			16	21
V Unskilled			13	17
Education	2 . 30	. 98	92	100
I GCSE/equivalent or below			25	27
II A level/equivalent			23	25
III Undergraduate degree			35	38
IV Postgraduate degree			9	10

Note: Full sample statistics are indicated in bold. Occupation information unavailable for 14 participants; education data lists maximum qualification obtained (or in progress). Typically, school leaving age of 16 corresponds to level I, 18 to level II; levels III and IV included participants currently undertaking that level of study. WAIS-IV, Wechsler Adult Intelligence Scale – Fourth Edition.

Table 2. Correlations and direct comparison among hold and no hold measures.

Correlations	Hold tests			Pairwise t-tests
	Vocabulary	Information	Combined
No-hold tests
Block Design	.29	.32	.34
Digit Span	.40	.23	.36
Combined (2 tests)	.49	.38	.49	Hold vs. no-hold (two tests): t(91) = 2.16, p = .034
Arithmetic	.43	.32	.42
Coding	.39	.34	.41
Combined (4 tests)	.58	.46	.59	Hold vs. no-hold (four tests): t(91) = 2.34, p = .021

Note: p values not corrected for multiple comparisons.

Table 3. NART and WTAR raw error and predicted and observed WAIS-IV performance.

	Range	Mean	SD	Sig. diff.
NART errors (max = 50)	2–46	18.30	8.98	p < .001
NART predicted FSIQ	81–124	108.52	8.78
WTAR errors (max = 50)	1–34	9.10	6.97
WTAR predicted FSIQ	78–120	108.53	8.51
Actual WAIS-IV performance
FSIQ	80–150	108.52	12.71	p = .043
GAI	80–148	108.83	13.50
VCI	74–138	107.14	14.73
PRI	82–144	108.75	12.48
WMI	80–148	106.07	14.28
PSI	81–146	104.86	13.25

NART, National Adult Reading Test; WTAR, Wechsler Test of Adult Reading; WAIS-IV Wechsler Adult Intelligence Scale – Fourth Edition; FSIQ, WAIS-IV full-scale IQ; GAI, General Ability Index; VCI, Verbal Comprehension; PRI, Perceptual Reasoning; WMI, Working Memory; PSI, Processing speed.

Table 4. Correlations of NART and WTAR performance with WAIS-IV FSIQ, index and subtest scores.

Measure	NART	WTAR	NART + WTAR
Full Scale IQ	.69***	.67***	.70***
Global Ability Index	.64***	.62***	.65***
Verbal Comprehension Index	.66***	.68***	.69***
Similarities	.36***	.44***	.41***
Vocabulary	.75***	.75***	.78***
Information	.53***	.57***	.56***
Perceptual Reasoning Index	.45***	.39***	.44***
Block Design	.29**	.23*	.27**
Matrix Reasoning	.43***	.38***	.42***
Visual Puzzles	.35**	.31**	.34***
Working Memory Index	.50***	.47***	.50***
Digit Span	.45***	.41***	.46***
Arithmetic	.40***	.39***	.41***
Processing speed Index	.36***	.36***	.37***
Symbol Search	.28**	.30**	.30**
Coding	.39***	.37***	.39***

NART, National Adult Reading Test; WTAR, Wechsler Test of Adult Reading; WAIS-IV Wechsler Adult Intelligence Scale – Fourth Edition; FSIQ, WAIS-IV full-scale IQ; ***p < .001; **p < .01.

Figure 2. Linear correlation between National Adult Reading Test/Wechsler Test of Adult Reading (NART/WTAR) errors and Wechsler Adult Intelligence Scale – Fourth Edition (WAIS-IV) full-scale IQ (FSIQ). The original published estimates of WAIS (dotted) and WAIS-R FSIQ (wide-space dashed) from the manual (Nelson & Willison, 1991) are included for comparison.

Figure 3. Scatterplots showing linear correlations relating number of the National Adult Reading Test (NART) and Wechsler Test of Adult Reading (WTAR) errors to (A) General Ability Index (GAI); (B) Verbal Comprehension (VCI); (C) Perceptual Reasoning (PRI); and (D) Working Memory (WMI). Processing speed (PSI) has been excluded.

Table 5. Linear regression models incorporating test scores (NART, WTAR) and demographic variables as predictors of WAIS-IV FSIQ performance.

Model	NART	WTAR	Age	Education	R^2
NART, demographics
1	1	0	0	0	. 48
2	1	0	1	0	. 53
3	1	0	0	1	.51
4	1	0	1	1	.54
WTAR, demographics
5	0	1	0	0	. 45
6	0	1	1	0	.46
7	0	1	0	1	. 50
8	0	1	1	1	.50
NART, WTAR, demographics
9	1	1	0	0	. 49
10	1	1	1	0	. 54
11	1	1	0	1	.55
12	1	1	1	1	.55
Demographics only
13	0	0	1	0	. 07
14	0	0	0	1	. 18
15	0	0	1	1	.27

NART, National Adult Reading Test; WTAR, Wechsler Test of Adult Reading; WAIS-IV Wechsler Adult Intelligence Scale – Fourth Edition. FSIQ, WAIS-IV full-scale IQ; Note: 1 = included in model; 0 = excluded from model. Bold values indicate significant single predictor models and stepwise multivariate models in which the fit is significantly improved.

Table 6. Single test (model 1) and combined (model 2) example estimates of WAIS-IV FSIQ.

NART errors	Model 1	Model 2
		Age (years)
		20	45		70
0	126	135	131		123
25	102	105	99		93
50	78	73	68		62
WTAR errors	Model 1	Model 2
		Education level
		4	3	2	1
0	120	123	120	118	115
25	89	96	93	90	87
50	59	69	66	63	60

NART, National Adult Reading Test; WTAR, Wechsler Test of Adult Reading; WAIS-IV, Wechsler Adult Intelligence Scale – Fourth Edition. FSIQ, WAIS-IV full-scale IQ; Note: Education level 1 = GCSE/equivalent or below; 2 = A level/equivalent; 3 undergraduate degree; 4 postgraduate degree.

Nelson, H. E., & Willison, J. (1991). National Adult Reading Test (NART). Windsor: NFER-Nelson.

 Google Scholar

Wechsler, D. (2001). Wechsler Test of Adult Reading: WTAR. San Antonio, TX: Psychological Corporation.

 Google Scholar

Pearson, N. C. S. (2009). Advanced clinical solutions for WAIS-IV and WMS-IV: Administration and scoring manual. San Antonio: The Psychological Corporation.

 Google Scholar

Wechsler, D. (2011). Test of Premorbid Functioning. UK version (TOPF UK). London: Pearson Assessment.

 Google Scholar