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The bias (an estimate of systematic error) is used as the measure of trueness (Table 3). The targets regarding trueness are stated so that the bias (average of O-B over a certain period) should be close to zero for all measured variables (sections 3.1 and 3.2). The trueness is assessed for all the temporal intervals considered in the tool (section 4): 6-hourly, daily and, in the future, monthly.  Also, a 5-day moving average (Alert) of the absolute value of daily calculated O-B (Table 9) needs to be calculated daily for all observed variables and compared against the prescribed thresholds (Table 6). This is used as one of the main performance indicators on the daily monitoring activities. 

Table 3 - Trueness

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Table3 Table3

Table 3 - Trueness

Precision 

The standard deviation (estimate of random error) is the quantitative measure of precision (Table 4). The targets for precision are applied to the standard deviation of O-B over a certain period for each of the observed variables (Table 6).  Like trueness, precision will be assessed 6-hourly, daily and monthly. Also, the 5-day moving average (Alert) of daily calculated standard deviation of O-B (Table 9) will be calculated for all variables and compared to the respective prescribed threshold ( Table 6).  This together with the performance indicator for Trueness will be used by the Evaluation function on their daily monitoring activities to determine the level of priority for stations showing accuracy/measurement uncertainty issues (see table in Annex2 of WDQMS Guidance Document). 

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Table4 Table4

Table 4 - Precision

Both bias and standard deviation contribute to the overall measurement uncertainty. The root mean square error (rmse) is a common metric used to measure accuracy, and it is applied to upper-air observations in the WDQMS web tool (section 6.2) based on the quantitative information for the two vertical layers (mean and standard deviation of O-B departures over Trop and Stra, see section 2.2)  provided by the NWP reports (see Table 5).

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Table5 Table5

Table 5 - Root Mean Square Error (rmse)

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Whenever the WDQMS performance targets are revised, the targets defined in WMO RRR for the global NWP application area are taken into account.  Table 7 contains the prescribed exceedance limits of measurement uncertainty for the variables monitored by WDQMS as defined by WMO RRR.

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Table6 Table6

Table 6 - Exceedance thresholds of trueness and precision as defined in the WDQMS Guidance Document (WMO, 2018).

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Table7 Table7

Table 7 - Exceedance limits of measurement uncertainty (combination of trueness and precision) for the relevant surface and upper-air variables as defined by WMO RRR (https://www.wmo-sat.info/oscar/requirements).

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