Analyses of covariance with baseline values ( pain or disability ) as covariates showed that transcranial direct current stimulation was ineffective for the reduction of pain ( difference between groups on visual analogue scale 1 mm ( 99 % confidence interval -8.69 mm to 6.3 mm ; P = 0.68 ) ) and disability ( difference between groups 1 point ( -1.73 to 1.98 ; P = 0.86 ) ) and did not influence the outcome of cognitive behavioural management ( difference between group 3 mm ( -10.32 mm to 6.73 mm ) ; P = 0.58 ; difference between groups on Oswestry disability index 0 point ( -2.45 to 2.62 ) ; P = 0.92 ) .