Detection of Insincere Grips:Multivariate Analysis Approach

Dr Bhoomiah Dasari

University of Southampton

Southampton SO17 1BJ

United Kingdom

bdd@soton.ac.uk

Introduction

• A standardized grip strength assessment can provide us with quantifiable and objective information on client’s hand functions (Kuzala & Vargo, 1991)

• Measurement of grip strength is a basis to use in the physical medicine to assess patient’s work capacities and progress of rehabilitation (Gilbert & Knowlton, 1983).

• Functional Hand evaluation is a kind of performance test, its validity, therefore relies heavily on the cooperation or sincerity of the subjects being assessed.

Previous studies of Grip sincerity

• Static Isometric Grip Test Approach (Stokes 1983, Stoke et. al 1995, Niebuhr and Marion 1987, Niebuhr and Marion 1990)

• Rapid Alternate Grip Test Approach (Hildreth, Breidenbach, Lister and Hodges 1989, Joughin et al 1993)

• Coefficient of Variation Approach (Bechtol 1954, Robinson et al 1993, Fairfax, Balnave & Adams 1995)

• Sustain Grip Test Approach (Kroemer & Marras 1980, Gilbert and Knowlton 1983, Smith et al 1989, Chengalur et al 1990)

Chenglaur’s Method (1990)

• The EVAL system by Greenleaf Medical Systems was used in this research

Typical Force-Time Curve For Sincere & Fake Condition

0

5

10

15

20

25

30

35

400

0.5 1

1.5 2

2.5 3

3.5 4

4.5 5

Time (Sec)

Gri

p S

tren

gth

(L

bs)

Fake Trial

Sincere Trial

P=Peak forceA=average of plateauSD=SD of plateau

The discriminators (Chengalur,1990)

Derived from data obtained from the sustain grip test• D1. Ratio: 100 * A / P (greater – more sincere)

• D2. Coefficient of Variation: 100 * SD of Plateau / Mean of Plateau (smaller – more sincere)

• D3 called “Ratio Difference”. It was devised for comparison between major & minor hand (Maj / Min) characteristics. (for healthy subjects only: greater – more sincere)

• D4: Peak-Average Difference: {(P-A) * 100 }/ (P *A) (smaller – more sincere)

• D5: Peak-Average Root Difference: (smaller – more sincere)

82

4410*

)*( AP

AP

Criterion value (method proposed by Chengalur,1990)

• *Remarks: Stand for the cut-off point using 95% confidence level

(If subject has D4 score equal or less than the cut-off point, it is 95% sure the subject lies within the sincere group)

D4 Score for Sincere And Fake Trial – hypothetical (smaller – more sincere)(smaller – more sincere)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

00.

380.

751.

13 1.5

1.88

2.25

2.63 3

3.38

3.75

4.13 4.

54.

885.

25

D4 Score

Fre

qu

en

cy

SincereTrialFake Trial

Zone I Zone II

cut-off point

Objectives of current study

●To test the applicability of Chengalur’s methods and its findings in Chinese subjects.

●To explore the use of multivariate analysis (logistic regression) on the raw data obtained in the sustain grip test to detect faking.

Mean (S.D.) n Age (yr) Height (cm) Weight (kg)Male 20 31.1.(18.0) 167.8 (11.9) 63.3 (28.4)Female 20 26.8 (10.7) 157.7 (4.6) 48.5 (9.3)Total 40 28.9 (14.8) 162.7 (10.3) 55.9 (22.2)

All subjects are all right hand dominance

Demographic Profile of Healthy Subjects (Group 1)

Education LevelCount (%) Male Female TotalPrimary sch. 1 (5.0) 0 (0.0) 1 (2.5)Secondary 3 3 (15.0) 2 (10.0) 5 (12.5)Secondary 5 1 (5.0) 5 (25.0) 6 (15.0)Matriculate 8 (40.0) 6 (30.0) 14 (35.0)University 7 (35.0) 7 (35.0) 14 (35.0)

Demographic Profile of Healthy Subjects (Group 1)

Mean (S.D.) n Age (yr) Height (cm) Weight (kg)Male 30 42.1 (14.3) 168.2 (7.4) 64.9 (11.8)Female 50 52.3 (12.0) 153.9 (13.0) 56.2 (9.1)Total 80 48.5 (13.8) 159.3 (13.1) 59.4 (11.0)

Demographic Profile of Ex-hand Injured Patients (Group 2)

Education LevelCount (%) Male Female Totalilliterate 1 (3.3) 1 (2.0) 2 (2.5)Primary sch. 5 (16.7) 30 (60.0) 35 (43.8)Secondary 3 12 (40.0) 7 (14.0) 19 (23.8)Secondary 5 5 (16.7) 8 (16.0) 13 (16.3)Matriculate 3 (10.0) 2 (4.0) 5 (6.3)University 4 (13.3) 2 (4.0) 6 (7.5)

Hand dominanceCount (%) Male Female TotalRight 29 (96.7) 50 (100) 79 (98.8)Left 1 (3.3) 0 (0) 1 (1.3)

Demographic Profile of Ex-hand Injured Patients (Group 2)

Test Procedure

• All subjects are taught to do the sustain grip test using the EVAL System.

• All subjects were randomized into 2 subgroups – subgroup 1a perform sincere grip and then fake grip, while subgroup 1b perform fake grips first and then sincere grip, etc.

• For fake grips, subjects are assigned randomly a faking ratio of 25%, 50%, or 75% of maximum grip strength.

Test Procedure

• Findings from sustain grip tests were converted into the discriminator, i.e. D1, D2, D4, D5. (D3 was used to healthy subjects only).

• % of accurate detection of sincere and fake group by using the Chengular’s method and Logistic regression was estimated and compared.

Instrumentation • The EVAL system by Greenleaf Medical Systems was

used in this research

Typical Force-Time Curve For Sincere & Fake Condition

0

5

10

15

20

25

30

35

400

0.5 1

1.5 2

2.5 3

3.5 4

4.5 5

Time (Sec)

Gri

p S

tren

gth

(L

bs)

Fake Trial

Sincere Trial

P=Peak forceA=average of plateauSD=SD of plateau

Hypothetical distribution of D4 showing the application of criterion value in determining fake and sincere group

Remarks: the cut-off pointZone I: Under the blue curve - true sincere grip (87.5%)

Under the red curve - false sincere grip (type II error, 21.2%)Zone II: Under the blue curve - false fake grip (type I error, 12.5%)

Under the red curve - true fake grip (77.8%)

Hypothetical distribution of D4 (male)

0

D4 Value

Fre

qu

en

cy

SincereGripFake Grip

Zone I Zone II

1.10

Percentage of successful detection of fake grip by Discriminant Analysis

(Group 1-healthy subjects)

Enter method Stepwise methodPredictionaccuracy D1-5 D4

Male 66.7% 69.4%Female 52.6% 39.5%

Percentage of successful detection of fake grip by Logistic Regression

(Group 1-healthy subjects)

Predictionaccuracy

Enter method Stepwise method

D1-5 D2, D4 D1, D3, D4Male 77.8% 77.8%Female 86.8% 86.8%

Percentage of successful detection of fake grip by Discriminant Analysis

(Group 2-Ex-patients)

Enter method Stepwise methodPredictionaccuracy D1-D5 D4-D5 D1, D4-D5

Male 60.0% 56.7%Female 59.0% 62.0%

Enter method Stepwise methodPredictionaccuracy D1-5 D2, D4 D1, D4Male 88.3% 88.3%Female 77.0% 76.0%

Percentage of successful detection of fake grip by Logistic Regression

(Group 2-Expatients)

Percentage of successful detection of fake grip by Discriminant

Analysis (Groups 1 & 2)

Enter method Stepwise methodPredictionaccuracy D1-5 D4-D5 D1, D4-D5

Male 62.5% 59.4%Female 55.8% 53.6%

Enter method Stepwise methodPredictionaccuracy D1-5 D2, D4 D1, D4

Male 83.3% 82.3%Female 78.3% 79.7%

Percentage of successful detection of fake grip by Logistic

Regression (Groups 1 & 2)

Results-Summary• Both methods, i.e. Discriminant Analysis and Logistic Regression can detect between 90%-

100% sincere grips.

• For subjects, who performed at 20% or 50% of their maximum grips, it was possible to detect 92% faking in males and 86.5% in female by using logistic regression method.

True sincere True sincere True fakingTrue faking

Conclusions

• It is possible to obtain better prediction accuracy by using the multivariate statistical method of approach (Logistic regression).

• It is also possible to formulate a mathematical model for detecting the faking grip.

• This, however will depend on a larger sample for developing a more reliable formula for clinical and medico-legal application.