Dear Collective

Old chestnut, but I am aware that some trusts are having issues of interpretation of how they are deriving their MoU’s

 

I have found the official UKAS paper on this (attached)

http://www.ukas.com/library/Technical-Information/Pubs-Technical-Articles/Pubs-List/LAB12.PDF

if your trust blocks attachments

In that document there is a completely unambiguous statement about the standard uncertainty unit

STANDARD UNCERTAINTY

7.2.1

The standard uncertainty is defined as one standard deviation. The potential

for mistakes at a later stage of the evaluation may be minimised by

expressing all uncertainty components as one standard deviation. This may

require adjustment of some uncertainty values, such as those obtained from

calibration certificates and other sources, which often will have been

expressed to a higher level of confidence, involving a multiple of the standard

deviation”

 

My understanding is that you need to combine individual MoU’s to derive an overall figure. For all practical purposes that means combinations of SD’s at different control levels

Example below

Our  ALT assay 1sd low control = 2.12

1 sd high control = 4.9

 

Then using the standard combination formula

 

1.96 *  sqr rt( 2.12 (squared) + 4.9 (squared))

 

MoU = 11 (rounded)

 

I am fairly sure that this view is totally over simplistic otherwise how could UKAS fill a 3 day course costing over £1000 to address the issue.

http://www.ukas.com/services/Training/Courses/Uncertainty_of_Measurement.asp

Thoughts please

 

BW John