Root mean square z_d,rms (blue solid line) and standard deviation σ_zd (orange solid line) of the vertical dust particle positions as functions of time, considering only the time after dust settling. The mean of each quantity is shown as a dashed line. The left and right panel depict simulations of the vertical shear instability and the streaming instability with 3 cm-sized dust. The average root mean square is more than an order of magnitude greater than the standard deviation in former simulation, while they are similar in the latter simulation.