NN intervals calculated over short periods, usually 5 minutes. SDANN is therefore a measure of changes in heart rate due to cycles longer than 5 minutes. SDNN reflects all the cyclic components responsible for variability in the period of recording, therefore it represents total variability. RMSSD ("root mean square of successive differences"), the square root of the mean of the squares of the successive differences between adjacent NNs. SDSD ("standard deviation of successive differences"), the standard deviation of the successive differences between adjacent NNs. NN50, the number of pairs of successive NNs that differ by more than 50 ms. pNN50, the proportion of NN50 divided by total number of NNs. NN20, the number of pairs of successive NNs that differ by more than 20 ms. pNN20, the proportion of NN20 divided by total number of NNs. EBC ("estimated breath cycle"), the range (max-min) within a moving window of a given time duration within the study period. The windows can move in a self-overlapping way or be strictly distinct (sequential) windows. EBC is often provided in data acquisition scenarios where HRV feedback in real time is a primary goal. EBC derived from PPG over 10-second and 16-second sequential and overlapping windows has been shown to correlate highly with SDNN. Geometric methods The series of NN intervals also can be converted into a geometric pattern such as: Geometric Measures HRV triangular index: integral of density distribution / maximum of density distribution maximum HRV triangular index = Number of all NN intervals / maximum number. Dependent on the length of the bin -> quote the bin size+ relative insensitive to the analytic quality of the series of NN intervals – need of reasonable number of NN intervals to generate the geometric pattern (in practice 20 min to 24 h) – not appropriate to assess short-term changes in HRV the sample density distribution of NN interval durations; sample density distribution of differences between adjacent NN intervals; a scatterplot of each NN (or RR) interval with the immediately preceding NN (or RR) interval – also called "Poincare plot" or (apparently in error) a "Lorenz plot"; and so forth. A simple formula is then used that judges the variability on the basis of the geometric and/or graphics properties of the resulting pattern. Frequency-domain methods Frequency domain methods assign bands of frequency and then count the number of NN intervals that match each band. The bands are