It is possible to propagate statistical errors through an analysis procedure via the use of an error array. Error arrays can be used in such tasks as imcnts, imcontour and imsmooth. An error array contains the squares of the errors (it is the data file itself for Gaussian statistics).
With the release of PROS 2.3, library routines and tasks were changed so as to use Poisson statistics by default, rather than Gaussian statistics. For normal operation:
xs> errcreate ``xdata$snr.qp[bl=2]''
will make a 
array with 16
pixels from
the Einstein IPC image, SNR.qp. Each array element will contain the
square of the error. The output file will be snr_err.imh.
For Gaussian statistics, one needs to change the value of keyword POISSERR in the image header. (For more information, see Section 2.6.2 in this Guide; see also help errcreate and help explain_errors.