  We conduct molecular dynamics simulations of a bidisperse Kob-Andersen (KA)
glass former, modified to add in additional polydispersity. The original KA
system is known to exhibit dynamical heterogeneity. Prior work defined
propensity, the mean motion of a particle averaged over simulations
reconstructing the initial positions of all particles but with randomized
velocities. The existence of propensity shows that structure and dynamics are
connected. In this paper, we study systems which mimic problems that would be
encountered in measuring propensity in a colloidal glass former, where
particles are polydisperse (they have slight size variations). We mimic
polydispersity by altering the bidisperse KA system into a quartet consisting
of particles both slightly larger and slightly smaller than the parent
particles in the original bidisperse system. We then introduce errors into the
reconstruction of the initial positions that mimic mistakes one might make in a
colloidal experiment. The mistakes degrade the propensity measurement, in some
cases nearly completely; one no longer has an isoconfigurational ensemble in
any useful sense. Our results show that a polydisperse sample is suitable for
propensity measurements provided one avoids reconstruction mistakes.
