Primary outcome showed a mean compression depth of 44.1 mm , with an inter-individual standard deviation ( SDb ) of 13.0 mm and an intra-individual standard deviation ( SDw ) of 6.7 mm for the control group vs. 46.1 mm and a SDb of 9.0 mm and SDw of 10.3 mm for the intervention group ( difference : 1.9 ; 95 % confidence interval -6.9 to 10.8 ; p = 0.66 ) .