XMM-Newton Science Analysis System


ombackground (ombackground-0.7.5) [xmmsas_20230412_1735-21.0.0]

The Sn value

On top of some of the graphs you will notice that an Sn value is given- this parameter gives a robust estimate of the data spread (standard deviation), and is computed using

$Sn = cn \times 1.1926 \times LOMED_i HIMED_i \mid x_i-x_j \mid$

Please see the paper by Rousseeuw, P.J. and Croux, C. (1993), "Alternatives to the Median Absolute Deviation," Journal of the American Statistical Association, Vol. 88, 1273-1283.

For an image with pure poisson noise and few sources the Sn value should be roughly equal to the mean of all the image pixel values.