Final Report Report No.: 1 Program No.: WAT-CH-0001 Program ...

Report No.: 1
significant figure of the robust standard deviation and of the equivalent figure in the robust
average.
6-3- Standard deviation for proficiency assessment (SDPA)
The standard deviation used to assess the proficiency of participants in this round of scheme
is derived from the results reported by the participants in the round Two methods have been
used for determination of SDPA as follows:
Method 1: Robust standard deviation (s*)
The standard deviation is the robust standard deviation of the results (s*) reported by all the
participants, calculated using the algorithm in 6-2.
Method 2: IQR
The standard deviation is the interquartile range (IQR). IQR is a measure of the variability of the
results. It is equal to the interquartile range (IQR) multiplied by a correction factor of 0.7413,
which makes it comparable to a standard deviation. The interquartile range is the difference
between the lower and upper quartiles. The lower quartile (Q1) is the value below which, as
near as possible, a quarter of the results lie. Similarly the upper quartile (Q3) is the value above
which a quarter of the results lie. The IQR = Q3 – Q1 and the normalised IQR = IQR × 0.7413.
6-4- Measurement uncertainty of the assigned value (u x )
The standard uncertainty of the assigned value X is estimated as:
Where p is the number of participants.
6-5- Purpose of performance scoring
Once the assigned value for the parameters under test has been established, participant
laboratories are assessed on the difference between their result and the assigned value, with
this difference being represented by a performance score, normally the z-score. The
advantages of a z-score are:
- Results can be expressed in a form that is relatively easy to interpret and understand
- Results can be summarized in graphical or tabular form to depict overall performance
- A performance score allows participants to directly compare their own result with others
- If consistent statistical values are applied, a performance score enables participants to
monitor and trend their own performance over time.
6-6- z-scores
Z-score of the participants is calculated using the following formula:
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