Estimation of the observed rate of GRBs with 90-degree PA flips and the time nodes of these flips. Upper panel shows the normalized maximal post-flip polarized flux with different q by taking typical parameters of Γ = 300 and θ_j = 0.1. The bottom panel shows the normalized time at PA flip (t_obs, ch/t_peak) with different q. The gray regions represent that there are no PA flips in these areas.