  Using stacking of images obtained at different epochs, we studied the
variability properties of linear polarization of active galactic nucleus (AGN)
jets on parsec-scales. Our sample is drawn from the MOJAVE programme, and
consists of 436 AGNs manifesting core-jet morphology and having at least five
VLBA observing epochs at 15 GHz from January 1996 through August 2019, with
some additional archival VLBA data reduced by us. We employed a stacking
procedure and constructed maps of (i) standard deviation of fractional
polarization and electric vector position angle (EVPA) over epochs as the
measure of variability and (ii) median polarization degree to quantify typical
values in time. The distributions of these values along and across the jet were
analysed for the whole sample for the first time. We found that core EVPA
variability is typically higher than that of the jet, presumably due to
component blending and outflow bends in the core. The BL Lacertae object cores
have lower EVPA variability, compared to that of quasars, possibly due to lower
Faraday rotation measure, suggesting a stronger ordered magnetic field
component. The EVPA becomes more stable down the jet. Most of the sources
showing this trend have a time coverage of more than 12 years and at least 15
epochs. The possible cause could be the increase of stability in the magnetic
field direction, reflecting an increase in the fraction of the magnetic field
that is ordered. There are no significant optical-class-dependent or
spectral-class-dependent relations in the EVPA variability properties in AGN
jets.
