Estimation of measurement uncertainty in chemical analysis
10.2. Uncertainty component accounting for random effects
Estimating the within-lab reproducibility component of uncertainty
http://www.uttv.ee/naita?id=17913
https://www.youtube.com/watch?v=9-803-VViKo
The within-lab reproducibility (intermediate precision) component takes into account all uncertainty sources that are random in the long term (i.e. several months, preferably one year). So, quite some uncertainty sources that are systematic within a day will become random in the long term. The simplest way to find u (Rw) is from a number of repeated measurements of a control sample, organized, e.g. as a control chart. Alternatively, the pooled standard deviation approach as explained in section 6 can be used. If this is done then the u (Rw) can be found based on several different control samples, so that it will be an average value of all of them.
The number of values used for evaluation of u (Rw) must be sufficiently large. An initial estimate of u (Rw) can be obtained with 10-15 values but thereafter more data should be collected. Even more importantly, the time period during which the data are collected, must be sufficiently long (at least several months, preferably around a year) so that all the sources of variability in the procedure are taken into account. So, 10 values collected over a five-month time period is a better option than 20 values collected during 1.5 months.
Depending on situation u (Rw) can be used as absolute or relative value.
It is important to stress that u (Rw) should be estimated separately for different matrixes and different concentration levels.
