Procedure for finding TPC hit errors uses following creteria to select tracks:

Global tracks

Number of the fit points >=25

|1/Curvature| <=1./200

|Pt/cos(Dip)|>=0.5

Since in the recent data amount of pile-up tracks is significatly higher compared to the times when the calibration procedure was developed.

It may be usufull add some cuts to suppress pile-up tracks. Some suggestions:

Use only gloabal tracks associated to primary tracks from the best vertex. Restrict Z position of the primary vertex.

Please find below the effect of the requirement mHitsG.mVertex==1

no vertex restrictions                                                                       

a0i=0.002196664
a1i=0.002246431
a2i=0.05652036
a3i=0.006201989
a4i=0.001913506
a5i=0.0215189
a0o-0.001615286
a1o=0.000312303
a2o=0.06295392
a3o=0.006065889
a4o=1e-09
a5o=0.04890244

mHitsG.mVertex==1

a0i=0.0007572124
a1i=0.002773429
a2i=0.03022654
a3i=0.003429027
a4i=0.004444034
a5i=0.0140386
a0o=0.0008804669
a1o=0.0009524381
a2o=0.06073751
a3o=0.003329919
a4o=0.001016549
a5o=0.04783038