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Table 4 Comparison of original and DIF-free SF-36 subscales by grouping factors (t-value and effect size)

From: Exploring differential item functioning in the SF-36 by demographic, clinical, psychological and social factors in an osteoarthritis population

SF-36 sub-scale Age Gender Social class Social dep Employment status Mood BMI No joints Hip/Knee
Phys          
 Original 4.80*** 3.71*** 3.82*** 3.50*** 8.53*** 7.63*** 7.17*** 1.39 ns 0.09 ns
 No DIF 4.89*** 3.24***     7.30*** 7.14***   
 Effect size p = 0.015 p < 0.0005     p = 0.21 ns p = 0.02   
Pain          
 Original -1.06 ns 1.93* 5.63*** 2.99*** 3.77*** 6.18*** 6.67*** -0.23 ns 1.35 ns
 No DIF    5.95***   ^2.72***/4.43***     
 Effect size    p = 0.09   p = 0.006/0.006     
Role Phys          
 Original 5.40*** 1.70 ns 3.27*** 2.27* 8.89*** 5.13*** 4.22*** 1.29 ns 0.34 ns
 No DIF 4.82***    1.81 ns 9.24***     0.16 ns
 Effect p = 0.02    p = 0.07 p = 0.002     p = 0.42
Social          
 Original 1.54 ns 2.61*** 4.13*** 4.09*** 5.37*** 12.68*** 4.69*** 0.23 ns -0.45 ns
 No DIF 2.17*   4.01***   6.20***     
 Effect p = 0.04   p = 0.37   p = 0.002     
Role Emot          
 Original 2.92*** 1.66 ns 3.27*** 3.57*** 4.81*** 8.98*** 3.16*** -0.12 ns 0.75 ns
 No DIF          1.22 ns
 Effect          p = 0.12
Mental          
 Original -2.55* 2.89*** 3.45*** 3.49** 0.04 ns 22.24*** 2.94*** -1.13 ns 0.12 ns
 No DIF   2.12* 2.88***       
 Effect   p = 0.003 p < 0.0005       
Vitality          
 Original 0.12 ns 2.55* 2.31* 3.15*** 2.95*** 12.29*** 4.96*** 0.26 ns -0.55 ns
 No DIF 0.14 ns     3.53***     
 Effect p = 0.22     p = 0.001     
Gen health          
 Original -2.56* -0.80 ns 4.42*** 4.34*** 2.42* 11.14*** 3.96*** -1.59 ns -1.25 ns
 No DIF       10.17***    
 Effect       p = 0.80    
  1. ***p < 0.01, **p < =0.01, *p < =0.05. ^ as both pain items exhibit DIF, each is taken out and t-test run. Social Dep: social deprivation (Townsend) score.
  2. Boldface=different conclusion reached if measure DIF-free.