INTRO TO STATS Flashcards
One of the uses of statistics in analytical chemistry is therefore to.
Provide an estimate of
the likely value of that error; in other words, to establish the uncertainty associated with
the measurement.
The closeness of a result to
the true value
ACCURACY
The extent to which results
agree with one another
PRECISION
ACCURACY
Can be applied to a single
measurement, but is more
commonly applied to the mean
value of several repeated
measurements, or replicates.
PRECISION
Inherently related to
the standard deviation of the
repeated measurements.
Error
-Formally defined as the result of the measurement minus
the true value, (xi − μ)
■has both sign and units
■ further categorized in terms of their origin and effect on the
measured result: Systematic, random, and gross errors.
-Errors that always have the same
magnitude and sign, resulting in
a bias of the measured values
from the true value.
-Affect the accuracy of the final result
Systematic errors
(determinate errors)
-Have different magnitudes and signs,
and result in a spread or dispersion of
the measured values from the true
value.
-Affect the precision of the final result;
they may also affect accuracy if the
number of replicates used is too
small.
Random errors
(indeterminate errors)
Errors that are so serious (i.e. large
in magnitude) that they cannot be
attributed to either systematic or
random errors associated with the
sample, instrument, or procedure
-A ________ error in a set of
readings or measurements is
termed an outlier.
Gross errors
These cause replicate results to
differ from one another, so that the individual results
fall on both sides of the average value (10.10 ml in
this case). _______ errors affect the precision, or
repeatability, of an experiment. In the case of
student A it is clear that the random errors are small,
so we say that the results are precise.
Random errors
These cause all the results to be
in error in the same sense (in this case they are all
too high). The total systematic error (in a given
experiment there may be several sources of
systematic error, some positive and others
negative)is called the bias of the measurement.
Systematic errors
Affect Precision- Repeatability and Reproducibility
Random Error
Produce Bias- An overall deviation of a result from the true value even when random errors are very small.
Systematic Error
Cause replicate results to fall on either side of a mean value.
Random Error
Cause all results to be affected in one sense only ,all too high or all too low
Systematic Error
