From XDSwiki

ΔCC1/2 is a quantity that detects datasets/frames which are non-isomorphous. As described in Assmann and Diederichs (2016), ΔCC1/2 is calculated with the σ-τ method. This method is a way to calculate the Pearson correlation coefficient for the special case of two sets of values (intensities) that randomly deviate from their true values. The σ-τ method is not influenced by a random number sequence as shown in Karplus and Diederichs (2012). For the σ-τ method CC1/2 is calculated for all datasets/frames, which will be called CC1/2_overall and CC1/2 is calculated for all datasets/frames except for one dataset i, which is omitted from calculations and denoted as CC1/2_i. The difference of the two quantities is ΔCC1/2.

[math]\Delta CC_{1/2}= CC_{1/2 overall}-CC_{1/2\_i} [/math]

If ΔCC1/2 is > 0 (CC1/2_overall is bigger than CC1/2_i) it means that by omitting dataset i from calculations a lower CC1/2 results. As we want to maximize CC1/2 the dataset is kept for calculations, it is improving the whole merged dataset. If Δ CC1/2 is < 0 (CC1/2_overall is smaller than CC1/2_i) it means that by omitting dataset i from calculations a higher CC1/2 results, which is why we want to exclude it from calculations, because it is impairing the whole merged dataset.


The ΔCC1/2 method is applicable for single frames, SSX data and SFX data. The program XDSCC12 calculates ΔCC1/2 for the isomorphous and anomalous signal for XDS_ASCII.HKL and XSCALE.HKL files. Exact description of calculation and implementation are found at CC1/2.