Intra-Individual Variability: OTBM by OTBM-SD
Transcript of Intra-Individual Variability: OTBM by OTBM-SD
06/20/2012 Rohling - Meta-analysis & Base Rates 1
342
0
.575
.330
.328
3.410
Count
Num. Missing
R
R Squared
Adjusted R Squared
RMS Residual
Regression Summary
Intra-Individual SD vs. Overall Test Battery Mean (OTBM)
Inclusion criteria: Study Grp from RIM Response-Meyers Data.dat (imported).svd
1 1950.921 1950.921 167.731 <.0001
340 3954.618 11.631
341 5905.539
DF Sum of Squares Mean Square F-Value P-Value
Regression
Residual
Total
ANOVA Table
Intra-Individual SD vs. Overall Test Battery Mean (OTBM)
Inclusion criteria: Study Grp from RIM Response-Meyers Data.dat (imported).svd
32.723 1.293 32.723 25.300 <.0001
-.197 .015 -.575 -12.951 <.0001
Coefficient Std. Error Std. Coeff. t-Value P-Value
Intercept
Overall Test Battery Mean (OTBM)
Regression Coefficients
Intra-Individual SD vs. Overall Test Battery Mean (OTBM)
Inclusion criteria: Study Grp from RIM Response-Meyers Data.dat (imported).svd
0
5
10
15
20
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35
Intr
a-I
ndiv
idual S
D
40 50 60 70 80 90 100 110 120
Overall Test Battery Mean (OTBM)
Y = 32.723 - .197 * X; R^2 = .33
Regression Plot
Inclusion criteria: Study Grp from RIM Response-Meyers Data.dat (imported).svd
Intra-Individual Variability:
OTBM by OTBM-SD
Mercy Medical Center in Sioux City, IA
TBI Cases from John Meyers (MNB-RIM)
06/20/2012 Rohling - Meta-analysis & Base Rates 2
131
0
.56
.32
.31
3.17
Count
Num. Missing
R
R Squared
Adjusted R Squared
RMS Residual
Regression Summary
OTBM SD ss vs. OTBM ss
Row exclusion: Summary Scores.svd
1 605.27 605.27 60.17 <.0001
129 1297.58 10.06
130 1902.85
DF Sum of Squares Mean Square F-Value P-Value
Regression
Residual
Total
ANOVA Table
OTBM SD ss vs. OTBM ss
Row exclusion: Summary Scores.svd
35.084 2.554 35.084 13.739 <.0001
-.208 .027 -.564 -7.757 <.0001
Coefficient Std. Error Std. Coeff. t-Value P-Value
Intercept
OTBM ss
Regression Coefficients
OTBM SD ss vs. OTBM ss
Row exclusion: Summary Scores.svd
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5
10
15
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OT
BM
SD
ss
40 50 60 70 80 90 100 110 120
OTBM ss
Y = 35.08 - .21 * X; R^2 = .32
Regression Plot
Row exclusion: Summary Scores.svd
Intra-Individual Variability:
OTBM by OTBM-SD
RI Hospital Brown University
TBI Cases from Tremont
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0
5
10
15
20
25
30
OT
BM
-SD
40 50 60 70 80 90 100 110 120
OTBM
Intra-Individual Variability:
OTBM by OTBM-SD
Sarasota, FL & Detroit, MI – TBI Cases from
Philip Leithen & Geoffrey Kantor (MNB-RIM)
06/20/2012 Rohling - Meta-analysis & Base Rates 4
0
5
10
15
20
25
30
OT
BM
SD
40 50 60 70 80 90 100 110 120
OTBM
Y = 39.83 - .24 * X; R^2 = .3
Regression Plot
Row exclusion: Green IISD.svd
891
0
.55
.30
.30
4.60
Count
Num. Missing
R
R Squared
Adjusted R Squared
RMS Residual
Regression Summary
OTBM SD vs. OTBM
Row exclusion: Green IISD.svd
1 8064.00 8064.00 380.89 <.0001
889 18821.43 21.17
890 26885.43
DF Sum of Squares Mean Square F-Value P-Value
Regression
Residual
Total
ANOVA Table
OTBM SD vs. OTBM
Row exclusion: Green IISD.svd
39.83 1.17 39.83 34.09 <.0001
-.24 .01 -.55 -19.52 <.0001
Coefficient Std. Error Std. Coeff. t-Value P-Value
Intercept
OTBM
Regression Coefficients
OTBM SD vs. OTBM
Row exclusion: Green IISD.svd
Intra-Individual Variability:
OTBM by OTBM-SD
Edmonton, Alberta, Private Practice
Mixed Neurological Cases from Paul Green
06/20/2012 Rohling - Meta-analysis & Base Rates 5
0
5
10
15
20
25
30
Mean s
s S
D
40 50 60 70 80 90 100 110 120
Mean ss
Y = 32.02 - .18 * X; R^2 = .28
Regression Plot
114
0
.53
.28
.27
3.01
Count
Num. Missing
R
R Squared
Adjusted R Squared
RMS Residual
Regression Summary
Mean ss SD vs. Mean ss
1 388.88 388.88 43.01 <.0001
112 1012.63 9.04
113 1401.51
DF Sum of Squares Mean Square F-Value P-Value
Regression
Residual
Total
ANOVA Table
Mean ss SD vs. Mean ss
32.024 2.482 32.024 12.901 <.0001
-.182 .028 -.527 -6.558 <.0001
Coefficient Std. Error Std. Coeff. t-Value P-Value
Intercept
Mean ss
Regression Coefficients
Mean ss SD vs. Mean ss
Intra-Individual Variability:
OTBM by OTBM-SD
Oklahoma Health Sciences University
Mixed Neurological Cases from Russell Adams
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0
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10
15
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25
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SD
40 50 60 70 80 90 100 110 120
Mean
Y = 39.649 - .257 * X; R^2 = .321
Regression Plot
339
0
.57
.32
.32
5.28
Count
Num. Missing
R
R Squared
Adjusted R Squared
RMS Residual
Regression Summary
SD vs. Mean
1 4440.49 4440.49 159.00 <.0001
337 9411.82 27.93
338 13852.30
DF Sum of Squares Mean Square F-Value P-Value
Regression
Residual
Total
ANOVA Table
SD vs. Mean
39.65 2.04 39.65 19.46 <.0001
-.26 .02 -.57 -12.61 <.0001
Coefficient Std. Error Std. Coeff. t-Value P-Value
Intercept
Mean
Regression Coefficients
SD vs. Mean
Intra-Individual Variability:
OTBM by OTBM-SD
CNS Vital Signs
Xanax Drug Study from David Loring
06/20/2012 Rohling - Meta-analysis & Base Rates 7
0
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OT
BM
sd
40 50 60 70 80 90 100 110 120
OTBM
Y = 34.71 - .2 * X; R^2 = .34
Regression Plot
192
1
.58
.34
.33
5.47
Count
Num. Missing
R
R Squared
Adjusted R Squared
RMS Residual
Regression Summary
OTBM sd vs. OTBM
1 2890.03 2890.03 96.45 <.0001
190 5693.22 29.96
191 8583.25
DF Sum of Squares Mean Square F-Value P-Value
Regression
Residual
Total
ANOVA Table
OTBM sd vs. OTBM
34.711 1.797 34.711 19.319 <.0001
-.202 .021 -.580 -9.821 <.0001
Coefficient Std. Error Std. Coeff. t-Value P-Value
Intercept
OTBM
Regression Coefficients
OTBM sd vs. OTBM
Intra-Individual Variability:
OTBM by OTBM-SD
University of South Alabama – Analog Simulation
Study for CNS Vital from Martin Rohling
06/20/2012 Rohling - Meta-analysis & Base Rates 8
65
0
.43
.18
.17
3.54
Count
Num. Missing
R
R Squared
Adjusted R Squared
RMS Residual
Regression Summary
SD1 SS vs. OTBM1 SS
1 177.02 177.02 14.10 .0004
63 790.69 12.55
64 967.71
DF Sum of Squares Mean Square F-Value P-Value
Regression
Residual
Total
ANOVA Table
SD1 SS vs. OTBM1 SS
28.411 2.109 28.411 13.473 <.0001
-.105 .028 -.428 -3.756 .0004
Coefficient Std. Error Std. Coeff. t-Value P-Value
Intercept
OTBM1 SS
Regression Coefficients
SD1 SS vs. OTBM1 SS
0
5
10
15
20
25
30
SD
1 S
S
40 50 60 70 80 90 100 110 120
OTBM1 SS
Y = 28.41 - .11 * X; R^2 = .18
Regression Plot
Intra-Individual Variability:
OTBM by OTBM-SD
Buffalo, NY – Dementia & MCI
Cases from Michael Santa Maria
06/20/2012 Rohling - Meta-analysis & Base Rates 9
99
0
.010
1.061E-4
•
3.662
Count
Num. Missing
|R|
R Squared
Adjusted R Squared
RMS Residual
Regression Summary
OTBMss-SD vs. OTBMss
1 .138 .138 .010 .9194
97 1300.985 13.412
98 1301.123
DF Sum of Squares Mean Square F-Value P-Value
Regression
Residual
Total
ANOVA Table
OTBMss-SD vs. OTBMss
14.847 4.942 14.847 3.004 .0034
.005 .051 .010 .101 .9194
Coefficient Std. Error Std. Coeff. t-Value P-Value
Intercept
OTBMss
Regression Coefficients
OTBMss-SD vs. OTBMss
0
5
10
15
20
25
30
OT
BM
ss-S
D
40 50 60 70 80 90 100 110 120OTBMss
Y = 14.847 + .005 * X; R^2 = 1.061E-4
Intra-Individual Variability:
OTBM by OTBM-SD
University of Indianapolis – Normal College
Students from Brad Axelrod & Jacqueline Wall
06/20/2012 Rohling - Meta-analysis & Base Rates 10
Summary of descriptive
statistics of each sample.
Sample Source Sample Dx
Sample n
OTBM M
OTBM sd r p Intercept Slope R2
1. Mercy Medical Meyers’ MNB
TBI 342 84.3
(12.2) 16.1 (4.2)
.58 < .0001 32.7 -0.20 .33
2. RI Hosp Brown Geoff Tremont
TBI 148 93.3
(11.6) 15.5 (4.3)
.56 < .0001 35.1 -0.21 .32
3. MI/FL MNB-RIM Leithen/Kantor
TBI 136 91.0 (9.0)
17.8 (3.1)
.35 < .0001 29.0 -0.12 .12
4. Edmonton, CA Paul Green
Neuro Pseudo
891 92.6
(12.3) 17.2 (5.5)
.55 < .0001 39.8 -0.24 .30
5. Oklahoma HSU Russell Adams
Neuro Pseudo
114 88.9
(10.2) 15.9 (3.5)
.53 < .0001 32.0 -0.18 .28
6. CNS-Vital Signs David Loring
Drug Study
339 99.2
(14.1) 14.2 (6.4)
.57 < .0001 39.7 -0.26 .32
7. USA CNS-VS7 Logistic Regression
Simulator study
192 85.3
(19.3) 17.5 (6.7)
.58 < .0001 37.1 -0.20 .34
8. Buffalo, NY Michael Santa Maria
DAT MCI
65 73.5
(15.7) 20.7 (4.0)
.43 .0004 28.4 -0.11 .19
9. University of Indy B. Axelrod & J. Wall
Normal 99 97.3 (7.3)
15.3 (3.6)
.01 .9194 14.8 0.01 .0001
Summary Stats ALL 2091 91.8
(13.5) 16.6 (5.4) .54 < .0001 35.4 -0.20 .30
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1 448.98 448.98 32.11 <.0001 32.11 1.00
3 166.95 55.65 3.98 .0086 11.94 .84
3 4.74 1.58 .11 .9524 .34 .07
234 3271.87 13.98
DF Sum of Squares Mean Square F-Value P-Value Lambda Pow er
Validity2
LOC Sev Grp.4
Validity2 * LOC Sev Grp.4
Residual
ANOVA Table for Intra-Individual SD
47 14.0 3.7 .5
51 14.9 4.0 .6
39 15.6 3.8 .6
41 17.0 3.6 .6
10 17.5 3.5 1.1
14 18.5 3.7 1.0
20 18.4 3.8 .8
20 19.9 3.3 .7
Count Mean Std. Dev. Std. Err.
Valid, LOC < 1 hour
Valid, LOC = 1-24 hours
Valid, LOC = 1-6 days
Valid, LOC => 7 days
Exag, LOC < 1 hour
Exag, LOC = 1-24 hours
Exag, LOC = 1-6 days
Exag, LOC => 7 days
Means Table for Intra-Individual SD
Effect: Validity2 * LOC Sev Grp.4
0
2.5
5
7.5
10
12.5
15
17.5
20
22.5
Cell
Mean
Valid Exag
Cell
LOC => 7 days
LOC = 1-6 days
LOC = 1-24 hours
LOC < 1 hour
Interaction Bar Plot for Intra-Individual SD
Effect: Validity2 * LOC Sev Grp.4
-3.4 1.1 <.0001 S
Mean Diff. Crit. Diff P-Value
Valid, Exag
Fisher's PLSD for Intra-Individual SD
Effect: Validity2
Significance Level: 5 %
-1.1 1.3 .1171
-1.9 1.4 .0055 S
-3.3 1.4 <.0001 S
-.9 1.3 .1918
-2.3 1.3 .0008 S
-1.4 1.3 .0437 S
Mean Diff. Crit. Diff P-Value
LOC < 1 hour, LOC = 1-24 hours
LOC < 1 hour, LOC = 1-6 days
LOC < 1 hour, LOC => 7 days
LOC = 1-24 hours, LOC = 1-6 days
LOC = 1-24 hours, LOC => 7 days
LOC = 1-6 days, LOC => 7 days
Fisher's PLSD for Intra-Individual SD
Effect: LOC Sev Grp.4
Significance Level: 5 %
Intra-Individual Variability:
OTBM by OTBM-SD
Mercy Medical Center in
Sioux City, IA – TBI Cases
from John Meyers
• Main effect for Effort or
Invalidity, as measured
by 9 embedded SVTs
• Main effect of TBI
Severity indexed by
LOC time
• No significant
interaction
06/20/2012 Rohling - Meta-analysis & Base Rates 12
1 356.22 356.22 16.75 <.0001 16.75 .99
2 116.49 58.25 2.74 .0655 5.48 .53
2 58.68 29.34 1.38 .2525 2.76 .29
539 11460.97 21.26
DF Sum of Squares Mean Square F-Value P-Value Lambda Pow er
Fail WMT.2
PTA Groups
Fail WMT.2 * PTA Groups
Residual
ANOVA Table for Green's OTBM-SD
365 10.7 3.0 .2
59 10.6 2.0 .3
2 12.6 3.8 2.7
112 15.7 8.4 .8
6 15.8 5.8 2.4
1 27.0 • •
Count Mean Std. Dev. Std. Err.
Gen WMT, PTA < 1 hr
Gen WMT, PTA = 1-24
Gen WMT, PTA = 1-6 days
Exag WMT, PTA < 1 hr
Exag WMT, PTA = 1-24
Exag WMT, PTA = 1-6 days
Means Table for Green's OTBM-SD
Effect: Fail WMT.2 * PTA Groups
0
5
10
15
20
25
30
Cell
Mean
Gen WMT Exag WMT
Cell
PTA = 1-6 days
PTA = 1-24
PTA < 1 hr
Interaction Bar Plot for Green's OTBM-SD
Effect: Fail WMT.2 * PTA Groups
-5.1 .9 <.0001 S
Mean Diff. Crit. Diff P-Value
Gen WMT, Exag WMT
Fisher's PLSD for Green's OTBM-SD
Effect: Fail WMT.2
Significance Level: 5 %
.7 1.2 .2193
-5.6 5.2 .0379 S
-6.3 5.3 .0209 S
Mean Diff. Crit. Diff P-Value
PTA < 1 hr, PTA = 1-24
PTA < 1 hr, PTA = 1-6 days
PTA = 1-24, PTA = 1-6 days
Fisher's PLSD for Green's OTBM-SD
Effect: PTA Groups
Significance Level: 5 %
Intra-Individual Variability:
OTBM by OTBM-SD
• Main effect for Effort or
Invalidity, as measured
by the WMT
• Main effect of TBI
Severity indexed by
PTA time
• No significant
interaction
Edmonton, Alberta – TBI
Cases from Paul Green
06/20/2012 Rohling - Meta-analysis & Base Rates 13
1 1502.65 1502.65 44.03 <.0001 44.03 1.00
135 4606.82 34.12
DF Sum of Squares Mean Square F-Value P-Value Lambda Pow er
Grp.6 SVTlog
Residual
ANOVA Table for OTBM sd
Inclusion criteria: Exclude Cases from CNSVS NAN Abstract.svd
67 21.1 5.4 .7
70 14.5 6.2 .7
Count Mean Std. Dev. Std. Err.
Mal Log
Gen Log
Means Table for OTBM sd
Effect: Grp.6 SVTlog
Inclusion criteria: Exclude Cases from CNSVS NAN Abstract.svd
0
2.5
5
7.5
10
12.5
15
17.5
20
22.5
Cell
Mean
Mal Log Gen Log
Cell
Interaction Bar Plot for OTBM sd
Effect: Grp.6 SVTlog
Inclusion criteria: Exclude Cases from CNSVS NAN Abstract.svd
6.6 2.0 <.0001 S
Mean Diff. Crit. Diff P-Value
Mal Log, Gen Log
Fisher's PLSD for OTBM sd
Effect: Grp.6 SVTlog
Significance Level: 5 %
Inclusion criteria: Exclude Cases from CNSVS NAN Abstract.svd
Intra-Individual Variability:
OTBM by OTBM-SD
• Main effect for Effort or
Invalidity, as measured
by the WMT
USA – Analog Study for CNS
Vital from Martin Rohling
06/20/2012 Rohling - Meta-analysis & Base Rates 14
1. Low scores relatively common across all test batteries
2. Low scores depend on where you set your cutoff score
3. Low scores vary by number of tests administered
4. Low scores vary by the demographics of the examinee
5. Low scores vary by level of intelligence.
5 Psychometric Principles for
Abnormal Scores by Iverson
(2011) & Schretlen et al. (2008)
06/20/2012 Rohling - Meta-analysis & Base Rates 15
Mean ES distribution of actual mTBI patient that includes invalid cases
-2 -2 7 3
-2 -1 8 8
-1 -1 4 12
-1 0 15 12
0 1 14 8
1 1 2 3
1 2 0 1
Total 50 49
From (>=) To (<) Count Normal Count
-.50
.75
.11
50
-1.85
.93
-.38
-1.09
-.27
Mean
Std. Dev.
Std. Error
Count
Minimum
Maximum
Skewness
Kurtosis
Median
ES
1.362 48 14.210 <.0001
Mean Diff. DF t-Value P-Value
Genuine, Exaggerate
30 .044 .086 .294 .054
20 -1.317 .147 .383 .086
Count Mean Variance Std. Dev. Std. Err
Genuine
Exaggerate
0
3
6
9
12
15
18
Co
un
t
-2 -1 0 1 2ES
Mean ES
From
(>)
To
(<)
Freq
Dist
% of
Score
%
Norm
Ideal
Freq
-2.5 -2.0 7 14.0 6.7 3
-2.0 -1.5 8 16.0 8.7 4
-1.5 -1.0 4 8.0 16.0 8
-1.0 -0.5 15 30.0 18.6 10
-0.5 0.0 14 28.0 18.6 10
0.0 0.5 2 4.0 16.0 8
0.5 1.0 0 0.0 8.7 4
1.0 1.5 0 0.0 6.7 3
-- -- 50 100 100
06/20/2012 Rohling - Meta-analysis & Base Rates 16
0
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15
20
25
30
35
Pe
rce
nt
0 10 20 30 40 50 60 70 80 90 100 110 120Skew ed Data
Histogram
43.92
12.64
.40
6.00
72.00
159.66
.29
66.00
-.72
.56
47.00
51.00
43.86
12.75
.40
8.00
82.00
162.69
.29
74.00
-.23
-.42
45.00
45.00
Mean
Std. Dev.
Std. Error
Minimum
Maximum
Variance
Coef. Var.
Range
Skewness
Kurtosis
Median
Mode
Patient 1 Normal 1
Descriptive Statistics
0
5
10
15
20
25
30
35
Pe
rce
nt
0 10 20 30 40 50 60 70 80 90 100 110 120Normal Data
Histogram
Low Functioning Patients
Impaired vs. Normal
Patient 1
Impaired
Normal 1
Unimpaired
06/20/2012 Rohling - Meta-analysis & Base Rates 17
0
5
10
15
20
25
30
35
Pe
rce
nt
0 10 20 30 40 50 60 70 80 90 100 110 120Skew ed Data 2
Histogram
63.92
12.64
.40
26.00
92.00
159.66
.20
66.00
-.72
.56
67.00
15.00
71.00
63.86
12.75
.40
28.00
102.00
162.69
.20
74.00
-.23
-.42
65.00
20.00
65.00
Mean
Std. Dev.
Std. Error
Minimum
Maximum
Variance
Coef. Var.
Range
Skewness
Kurtosis
Median
IQR
Mode
Patient 2 Normal 2
Descriptive Statistics
High Functioning Patients
Impaired vs. Normal
Patient 2
Impaired
Normal 2
Unimpaired
0
5
10
15
20
25
30
35
Pe
rce
nt
0 10 20 30 40 50 60 70 80 90 100 110 120Normal Data 2
Histogram
06/20/2012 Rohling - Meta-analysis & Base Rates 18
Low, Normal, and High (Genuine) Premorbid Function without Impairment
p = .11 p = .61 p = .68
06/20/2012 Rohling - Meta-analysis & Base Rates 19
Low, Normal, and High (Genuine) Premorbid Function with Impairment
p < .0001 p < .0001 p < .0001
06/20/2012 Rohling - Meta-analysis & Base Rates 20
Low, Normal, and High (Exaggerating) Premorbid Function - Dubious Impairment
p < .0001 p < .0001 p < .0001
06/20/2012 Rohling - Meta-analysis & Base Rates 21
1 38.43 38.43 9.34 .0025 9.34 .88
1 97.45 97.45 23.68 <.0001 23.68 1.00
1 .52 .52 .13 .7222 .13 .06
239 983.61 4.12
DF Sum of Squares Mean Square F-Value P-Value Lambda Power
Sev.2 Grp
SVT.2 Grp
Sev.2 Grp * SVT.2 Grp
Residual
44 9.3 1.9 .3
31 10.6 2.0 .4
82 10.1 2.0 .2
86 11.6 2.1 .2
Count Mean Std. Dev. Std. Err.
Normal, Genuine
Normal, Exaggerate
Impaired, Genuine
Impaired, Exaggerate
0
2
4
6
8
10
12
Ce
ll M
ea
n
Normal Impaired
Cell
ExaggerateGenuine
Intra-Individual Variability (IIV)
06/20/2012 Rohling - Meta-analysis & Base Rates 22
Low scores are relatively
common across all test batteries
1. Average subjects can be expected to obtain a distribution of test scores in which approximately 16% will fall 1 SD below the mean.
2. The range of abnormal scores obtained by an average person might be as low as 8% or as high as 28%.
3. In fact, for the eight NAB normative subjects who Iverson (2010) identified as possibly suffering from impairment, on a battery of 25 test scores, the average number of abnormal scores per subject was 18%, with a minimum of 8% and a maximum of 28%.
06/20/2012 Rohling - Meta-analysis & Base Rates 23
Low scores depend on where
you set your cutoff score
4. For any particular battery administered to an average person, with abnormality defined as 1 SD below the normative mean, will result in 16% of the scores being labeled as abnormal, with a range from 8% to 28%.
5. Similarly, with abnormality defined as 1.5 SD below the normative mean will result in 7% of the scores being labeled as abnormal, w/ range from 3% to 14%.
6. Finally, with abnormality defined as 2.0 SD below the normative mean will result in 2% of the scores being labeled as abnormal, with a range from 1% to 5%.
06/20/2012 Rohling - Meta-analysis & Base Rates 24
Low scores vary by number of
tests administered
7. Multiple the number of test scores obtained by the percentage of scores expected to fall below the selected cutoff and you will obtain the number of expected abnormal scores.
8. Giving more tests results in more abnormal test
scores, but the percentage of abnormal test scores does not change by increasing the number of tests administered in the battery. More scores will increase the reliability and validity of the assessment.
06/20/2012 Rohling - Meta-analysis & Base Rates 25
Low scores vary by
demographics of the examinee
9. Higher levels of premorbid functioning will result in a lower frequency of obtained abnormal scores.
10. Lower levels of premorbid functioning will result in a higher frequency of obtained abnormal scores.
11. Demographic variables, such as age and education, influence levels of premorbid functioning.
06/20/2012 Rohling - Meta-analysis & Base Rates 26
Low scores vary by level of
premorbid intelligence
12. Higher levels of premorbid functioning will result in a lower frequency of obtained abnormal scores.
13. Lower levels of premorbid functioning will result in a higher frequency of obtained abnormal scores.
14. Demographic variables, such as age and education, influence levels of premorbid functioning.