3.4.1.1 Repeatability
Repeatability is the precision estimate obtained when measurement results are produced in one facility and
tests are performed on identical test items during a short interval of time by one operator using the same
equipment under conditions that are as constant as possible (e.g. incubation time and temperature). It can
be expressed as standard deviation (s), variance (s
2
), probability distribution function, etc for a suitable
number of measurements made under repeatability conditions. If the method of analysis involves
instrumental techniques the instrument repeatability needs to be determined in addition to method
repeatability (examples are provided below). Repeatability gives an indication of the short-term variation in
measurement results and is typically used to estimate the likely difference between replicate measurement
results obtained in a single batch of analysis. However, it underestimates the spread of results that can be
expected under normal operating conditions over the longer term
The repeatability standard deviation, variance, probability distribution function, etc must be determined with
at least 6 degrees of freedom. This can be achieved for example, by analysing 7 times in a series with one
test item (df=6), 4 times in a series with 2 test items (df=6), 3 times in a series with 3 test items (df=6), etc
(Zar, 1974).
Technical Note 17 - Guidelines for the validation and verification of quantitative and qualitative test methods
June 2012
Page 15 of 32
Instrumental repeatability may be determined by the injection of the standard solutions that are used to
prepare the working calibration curve as well as an incurred or fortified sample at each of the spike levels 7
times. These injections should be done in random order to minimise bias. Calculate mean, standard
deviation and percent relative standard deviation.
Method repeatability may be determined by preparing pools of sample material with levels of the analyte(s)
at or near the concentrations used for method recovery studies. This may be done by using incurred material
or by fortifying material (blank or incurred) with the required amount of the analyte(s). Replicate extracts are
prepared of each of these samples and analysed by one analyst on the same day. Calculate mean, standard
deviation and percent relative standard deviation.
For microbiological analyses, repeatability can also be assessed in those cases where samples are plated
out in duplicate.
Do'stlaringiz bilan baham: |