  Unbiased and precise mass calibration of galaxy clusters is crucial to fully
exploit galaxy clusters as cosmological probes. Stacking of weak lensing signal
allows us to measure observable-mass relations down to less massive halos halos
without extrapolation. We propose a Bayesian inference method to constrain the
intrinsic scatter of the mass proxy in stacked analyses. The scatter of the
stacked data is rescaled with respect to the individual scatter based on the
number of binned clusters. We apply this method to the galaxy clusters detected
with the AMICO (Adaptive Matched Identifier of Clustered Objects) algorithm in
the third data release of the Kilo-Degree Survey. The results confirm the
optical richness as a low scatter mass proxy. Based on the optical richness and
the calibrated weak lensing mass-richness relation, mass of individual objects
down to ~10^13 solar masses can be estimated with a precision of ~20 per cent.
