LaMotte SMART Spectro Spectrophotometer - Manual (Page 7)

SMART Spectro Operator’s Manual 2.11 7
Precision: Precision is the numerical agreement between two or more
measurements. 3 The precision can be reported as a range for a measurement
(difference between the min and max). It can also be reported as the standard
deviation or the relative standard deviation. It is a measure of how close together
the measurements are, not how close they are to the correct or true value. The
precision can be very good and the accuracy very bad. This is a useful measure
of the performance of a test method.
3. Skoog, D.A., West, D. M., Fundamental of Analytical Chemistry, 2nd ed.,
Holt Rinehart and Winston, Inc, 1969, p. 26.
Accuracy: Accuracy is the nearness of a measurement to the accepted or true
value.4 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 know amount of analyte in a determination of the analyte (i.e.
103.5 %). This is a useful measure and what most customers are interested in
when they want to know about the performance of a test method.
4. Skoog D.A., West D. M., Fundamental of Analytical Chemistry, 2nd ed., Holt
Rinehart and Winston, Inc, 1969, p. 26.
Resolution: Resolution is the smallest discernible difference between any two
measurements that can be made.5 For meters this is usually how many decimal
places are displayed. (i.e. 0.01). For titrations and various comparators it is the
smallest interval the device is calibrated or marked to (i.e. 1 drop = 10 ppm,
0.2 ppm for a DRT, or ±half a unit difference for an octaslide or color chart).
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. This is often done with
various comparators. One caveat is, 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, very bad! This is not a useful measure of the performance of a test
method.
5. Statistics in Analytical Chemistry: Part 7 – A Review, D. Coleman and
L Vanatta, American Laboratory, Sept 2003, P. 34.
Sensitivity: Sensitivity is the resolution based on how this term is used in
LaMotte catalogs. This term is not listed in any of the references. Sometimes it is
used for detection limit. It is a confusing term and should be avoided.
Repeatability: Repeatability is the within-run precision.
6 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.
6. Jeffery G. H., Basset J., Mendham J., Denney R. C., Vogel’s Textbook of
Quantitative Chemical Analysis, 5th ed., Longman Scientifi c & Technical,
1989, p. 130.