  All estimates of cluster mass have some intrinsic scatter and perhaps some
bias with true mass even in the absence of measurement errors for example
caused by cluster triaxiality and large scale structure. Knowledge of the bias
and scatter values is fundamental for both cluster cosmology and astrophysics.
In this paper we show that the intrinsic scatter of a mass proxy can be
constrained by measurements of the gas fraction because masses with higher
values of intrinsic scatter with true mass produce more scattered gas
fractions. Moreover, the relative bias of two mass estimates can be constrained
by comparing the mean gas fraction at the same (nominal) cluster mass. Our
observational study addresses the scatter between caustic (i.e., dynamically
estimated) and true masses, and the relative bias of caustic and hydrostatic
masses. For these purposes, we used the X-ray Unbiased Cluster Sample, a
cluster sample selected independently from the intracluster medium content with
reliable masses: 34 galaxy clusters in the nearby ($0.050<z<0.135$) Universe,
mostly with $14<\log M_{500}/M_\odot \lesssim 14.5$, and with caustic masses.
We found a 35\% scatter between caustic and true masses. Furthermore, we found
that the relative bias between caustic and hydrostatic masses is small,
$0.06\pm0.05$ dex, improving upon past measurements. The small scatter found
confirms our previous measurements of a highly variable amount of feedback from
cluster to cluster, which is the cause of the observed large variety of
core-excised X-ray luminosities and gas masses.
