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**Checkpoint**: this macros will allow one to reproduce the plots 45-48 from the note as well as tables 13-15 from the note

##### ABCD method

We estimate QCD background using ABCD method in order to improve our systematic uncertainty on the background estimation. ABCD method is very simple.

1) choose 2 variables: assume two variables are independent

2) assume the fraction should be same if there is no correlation: N_A / N_B = N_C / N_D

3) In our study, use two variables: sign of muon pair, muon isolation

4) QCD fraction in each region has a dependence. We produce the correction factor for each region: B, C, D

5) Produce N_B, N_C, N_D from data sample, and estimate N_A from them at the end (applying the correction factors)

In UserCode/Purdue/DYAnalysis/AnalysisMacros/ABCDmethod

QCDFrac.C: to produce correction factors for each region

ABCD2vari.C: to produce the ABCD results. The correction factors from the QCDFrac.C are plugged in this macro as an input.

##### ttbar data driven background estimation

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