Table 5.

Multivariable analysis of the short and long questionnaires and demographics on pain intensity

Model*	Variables included in model	Adjusted R2†	p value	Part R2‡	p value
Including long questionnaires§
Pain	Overall	0.33	< 0.001
	PCS-13			0.14	< 0.001
	Months since first pain experience			0.019	0.033
	Years of education			0.025	0.014
	Differences in diagnosis
	No other pain conditions			0.011	0.097
	Osteoarthritis compared with nonspecific arm pain			0.054	< 0.001
Including short questionnaires§
Pain	Overall	0.32	< 0.001
	PCS-4			0.13	< 0.001
	Months since first pain experience			0.018	0.037
	Years of education			0.023	0.019
	No other pain conditions			0.013	0.076
	Differences in diagnosis
	Osteoarthritis compared with nonspecific arm pain			0.045	0.001

* The predictors found in the bivariate analysis were inserted in a backward, stepwise regression; this yields a model with the predictors that determine most of the variability of the variables; ^†percentage of the overall variability in the dependent variable explained or accounted for by the independent variables in the model; ^‡part R² = the individual contribution of each variable to the adjusted R²; ^§the regressions were conducted with all significant predictors from the bivariate analysis; when the questionnaires were entered, the first regression inserted the full questionnaires (eg, PHQ-9, PCS-13, SHAI-18); the second regression (including short questionnaires) inserted the same predictors, but instead of using the full questionnaires (eg, PHQ-9, PCS-13, SHAI-18), the PHQ-2, PCS-4 and SHAI-5 were inserted; the comparison of the models with the short and long questionnaires gives us an indication of the criterion validity; PHQ-9 and -2 = nine- and two-item Patient Health Questionnaire; PCS-13 and -4 = 13- and four-item Pain Catastrophizing Scale.