We compare X-ray and caustic mass profiles for a sample of 16 massive galaxy clusters .
We assume hydrostatic equilibrium in interpreting the X-ray data , and use large samples of cluster members with redshifts as a basis for applying the caustic technique .
The hydrostatic and caustic masses agree to better than \approx 20 \% on average across the radial range covered by both techniques ( \sim [ 0.2 - 1.25 ] R _ { \text { 500 } } ) .
The mass profiles were measured independently and do not assume a common functional form .
Previous studies suggest that , at R _ { \text { 500 } } , the hydrostatic and caustic masses are biased low and high respectively .
We find that the ratio of hydrostatic to caustic mass at R _ { \text { 500 } } is 1.20 ^ { +0.13 } _ { -0.11 } ; thus it is larger than 0.9 at \approx 3 \sigma and the combination of under- and over-estimation of the mass by these two techniques is \approx 10 \% at most .
There is no indication of any dependence of the mass ratio on the X-ray morphology of the clusters , indicating that the hydrostatic masses are not strongly systematically affected by the dynamical state of the clusters .
Overall , our results favour a small value of the so-called hydrostatic bias due to non-thermal pressure sources .