Hi Shan, Nidp does not (to first order, anyway) depend on the step size used the data collection. The step size in k does dictate the highest frequency (R) that can be measured. This could, in principle, change the effective R range that could be used in the Nidp calculation, but it should never have a serious impact on it. Usually (at least in my experience), step sizes in k are around 0.05Angstroms-1 when collecting EXAFS data, which would give Rmax = pi / (2 * step_k) = 31.4 Angstroms. Of course, that is significantly larger R than we can hope to measure, but it does allow us to do "crude signal processing" steps like linear interpolation, re-binning of data as we shift E0 and so on without ruining the data below, say 15.7 Angstroms. So, for EXAFS it's probably dangerous to have a step size larger than 0.2Angstroms^-1, and not a great help to have step sizes smaller than 0.05Angstroms. For what it's worth, IFEFFIT/Artemis more or less insist on interpolating chi(k) onto a k-grid of 0.05Ang^-1. All that being said, I have not carefully tested how large the step size has to be to really impact the EXAFS below, say, 5Angstroms^-1. Cheers, --Matt