At level A the difference was 0.13 0.18 ; 95 % CI 0.04 to 0.26 mm ( P = 0.34 ) , at level B it was 0.08 0.23 ; 95 % CI -0.14 to 0.14 ( P = 0.61 ) , at level C it was 0.05 0.25 ; 95 % CI -0.01 to 0.31 mm ( P = 0.55 ) and at level D it was 0.13 0.27 ; 95 % CI -0.02 to 0.32 mm ( P = 0.67 ) .