Statistical and Technical Definitions Related to Product Specifications
Method Detection Limit (MDL):
“The method detection limit (MDL) is defined as the
minimum concentration of a substance that can be measured and reported with 99%
confidence that the analyte concentration is greater than zero and is determined from
analysis of a sample in a given matrix containing the analyte.”
1
Note that, “As Dr.
William Horwitz once stated, ‘In almost all cases when dealing with a limit of detection
or limit of determination, the primary purpose of determining that limit is to stay away
from it.’”
2
Accuracy:
Accuracy is the nearness of a measurement to the accepted or true value.
3
The accuracy can be expressed as a range, about the true value, in which a
measurement occurs (i.e. ±0.5 ppm). It can also be expressed as the % recovery of a
known amount of analyte in a determination of the analyte (i.e. 103.5 %).
Resolution:
Resolution is the smallest discernible difference between any two
measurements that can be made.
4
For meters this is usually how many decimal places
are displayed. (i.e. 0.01). Note that the resolution many change with concentration or
range. In some cases the resolution may be less than the smallest interval, if it is
possible to make a reading that falls between calibration marks.
A word of caution,
that resolution has very little relationship to accuracy or precision. The resolution will
always be less than the accuracy or precision but it is not a statistical measure of how
well a method of analysis works. The resolution can be very, very good and the
accuracy and precision can be very bad! This is not a useful measure of the
performance of a test method.
Repeatability:
Repeatability is the within-run precision.
5
A run is a single data set,
from set up to clean up. Generally, one run occurs on one day. However, for meter
calibrations, a single calibration is considered a single run or data set, even though it
may take 2 or 3 days.
Reproducibility:
Reproducibility is the between-run precision.
6
Detection Limit (DL):
The detection limit (DL) for the 2020 is defined as the
minimum value or concentration that can be determined by the meter, which is greater
than zero, independent of matrix, glassware, and other sample handling sources of
error. It is the detection limit for the optical system of the meter.
1
CFR 40, part 136, appendix B
2
Statistics in Analytical Chemistry: Part 7 – A Review, D. Coleman and L Vanatta,
American Laboratory, Sept 2003, P. 31.
3
Skoog, D.A., West, D. M.,
Fundamental of Analytical Chemistry
, 2
nd
ed., Holt Rinehart
and Winston, Inc, 1969, p. 26.
4
Statistics in Analytical Chemistry: Part 7 – A Review, D. Coleman and L Vanatta,
American Laboratory, Sept 2003, P. 34.
5
Jeffery G. H., Basset J., Mendham J., Denney R. C.,
Vogel’s Textbook of Quantitative
Chemical Analysis
, 5
th
ed., Longman Scientific & Technical, 1989, p. 130.
6
Jeffery G. H., Basset J., Mendham J., Denney R. C.,
Vogel’s Textbook of Quantitative
Chemical Analysis
, 5
th
ed., Longman Scientific & Technical, 1989, p. 130
STATISTICAL AND TECHNICAL DEFINITIONS
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