**PURPOSE**- Averages 2 or more files, computing measures of variance (per point, total, etc.). The average and the variance images are stored in output files. Alternatively can split image set into two subsets and compute averages and variances for the subsets (without overall average and variance files). Works for images and volumes. Example.

**SEE ALSO****AS S**[Average image/volume series, optional subsets, select outputs] **AS F**[Average images - find statistics & ' *'Q'*' factor map ||]

**USAGE**- .OPERATION: AS R

- .INPUT FILE NAME TEMPLATE (E.G. PIC****): PIC***

[Enter name template for the image series to be averaged. Astericks in the template will be replaced by numbers solicted next.].FILE NUMBERS OR SELECTION DOC. FILE NAME: 1-10,12

[Enter numbers of files within the series to be added. The file numbers can also be read from a selection document file where file numbers are contained in the first register (not the keys).].ALL or ODD-EVEN FILES (A/O): O

['A' - all images will be averaged,

'O' - two subset-averages will be calculated: one for odd-numbered images and one for even numbered images.] - If option 'A' was specified the following prompts appear:

.AVERAGE FILE: AVG001

[Enter name of output file receiving average.].VARIANCE FILE: VAR001

[Enter name of output file receiving variance map.]- Otherwise the following prompts appear:

.AVERAGE FILE FOR ODD-NUMBERED IMAGES: avg1

[Enter names of output file receiving subset 1 average.]

.VARIANCE FILE FOR ODD-NUMBERED IMAGES: var1

[Enter names of output file receiving subset 1 variance.].AVERAGE FILE FOR EVEN-NUMBERED IMAGES: avg2

[Enter names of output file receiving subset 2 average.]

.VARIANCE FILE FOR EVEN-NUMBERED IMAGES: var2

[Enter names of output file receiving subset 2 variance.]The following values are printed out:

Number of images

Number of elements

Total variance

Total std. deviation

Variance. per point

Std. deviation per point

Average offset = 0.0

Variance of average image

**NOTES**

- This operation does not subtract the average from
each image during the calculation of global average
and variance. Thus, the statistics calculated are
'Real' and the average offset is always zero. The
obsolete operation: 'AS' results in the variance
estimator which is usually
far 'too optimistic' - the variance obtained is too small
and cannot be used for statistical tests.
- Merged with operation "AS DC" July 1999

**SUBROUTINES**- ADDS

**CALLER**- UTIL2

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