CHEMISTRY 162
EXPERIMENTAL MEASUREMENT AND RELIABILITY
The quality of measurements in the chemical laboratory is directly related to the
reliability of experimental results. Typically, quality of a measurement is related to the
number of significant figures read from the measuring device. These measurements are
then manipulated in such a way as to maximize the reliability of the experimental result.
There are other means, other than significant figures, to determine the reliability of a
result, which will be discussed below.
ACCURACY AND PRECISION
All experimental measurements are subject to error. Error is defined as the difference
between a measured value and the “true” value of a property. Expressing the reliability
of a measurement in terms of its accuracy is not often possible, since there are relatively
few instances where the true value of a property is known. Counted numbers of objects
or events are true values; so are the rational or irrational numbers which appear in
mathematical formulas. For example the numbers 1 and п in the formula for the area of a
circle (Area = пr2) are known to any desired accuracy.
Since an experimental measurement is subject to error, a property determined by that
measurement can never have a true value in the same sense that t a counted number does.
However, in some cases a property does have a value which is accepted as true by the
scientific community. An accepted value may be defined. For example, the atomic
weight of the 12C isotope is assigned a value of 12 followed by a decimal point and then
an infinite number of zeros. An accepted value may also be the most probable value
derived from repeated and careful measurements. For instance, the accepted value for the
density of liquid ethyl alcohol is 0.7852g/mL at a temperature of 25ºC.
True or accepted values for most experimentally measured properties are unknown.
When dealing with such a property, and experimenter is unable to evaluate the accuracy
of this measurement and must express its reliability in terms of precision. The precision
of a measurement is obtained by repeating that measurement several times. If the several
measured values show a reasonable agreement with one another, then the measurement is
said to be precise. Consider four values for the density of a liquid measured by one
experimenter: 0.7854, 0.7850, 0.7847, and 0.7830 g/mL; and four values determined by a
second experimenter: 0.7856, 0.7850, 0.7844, and 0.7830 g/mL. Clearly, the first set of
measurements is more precise than the second.
Repeating a measurement several times is often impractical or inefficient and it is
sometimes necessary to estimate the precision. Frequently, this estimate is based on the
limiting precision of some instrument or other apparatus used in the measurement. When
the limiting precision of an instrument is not specified, it must be estimated by the
experimenter.
Measurements may be precise without necessarily being accurate. This situation arises
when there is a constant source of error which affects each measurement of a particular
property in the same way. Consider the measurements of liquid density mentioned
before. If the volume of the density bottle used in each measurement is 2% high, then
each density value (mass/volume) would be too low by a factor of 2%. However, in the
absence of such a consistent error, an experimenter generally assumes that the more
precise the measurement of some property, the greater the chance of its being reliable.
STATISTICAL EVALUATION OF DATA AND RESULTS
Many of the experiments in this laboratory require two or more measurements of the
same property. The average value derived from these replicate measurements is taken as
the best value of the property. Statistical methods are then employed to evaluate the
reliability of this best value as well as the reliability of each individual value. The
following presentation outlines certain concepts and definitions used in the statistical
treatment of experimental data and results.
Arithmetic mean: The mean or average for a set of measured values of some property is
the sum of the individual values, xi, divided by the total number of values, N
X1 +X2 + X3 ... + Xn
X=
N
Deviation: The deviation, di, of an individual value is the absolute value of the
difference between that value and the mean:
di = Xi - X
Average Deviation: The average deviation, d, is the sum of the deviations for the
individual values (without regard to sign) divided by the total number of values:
d1 + d2 + d3 + ... + dn
d=
N
Relative average deviation: Relative average deviation is expressed as the ratio of the
average deviation of the individual values to the arithmetic mean:
d
X 100 = relative average deviation (%, or pph)
x
d
x 1000 = relative average deviation (ppt)
x
Percentage relative error: Percentage relative error is an accuracy index in which is a
ratio of the absolute error in a measured value (or mean) to the true or accepted value of a
property is multiplied by 100:
measured value - true value
% relative error =
true value
Absolute error is of little statistical importance, relative error does have significance in
those situations where the true or accepted value of a property is known. The statistical
concepts described are illustrated here using the values of the liquid density previously
mentioned.
mean = x =
Density (g/mL)
0.7854
0.7850
0.7847
0.7849
3.1400
3.1400
= 0.7850
4
Deviation (di)
0.0004
0.0000
0.0003
0.0001
0.0008
0.0008
= 0.0002
d =
4
Relative average deviation =
d
x
Relative average deviation(%, or pph) =
d
x
= 0.0003
x 100 = 0.3 %
d
x 1000 = 30 ppt
x
From the above calculations, one would report that the density is equal to 0.7850 g/mL.
One also should indicate how reliable this answer is known. In other words, an index of
precision is needed to indicate the degree of uncertainty in the calculated result. In this
laboratory, the recommended indices of precision are the average deviation and the
relative average deviation. Therefore, the experimental density can be reported in two
ways:
1) Using the average deviation, the value reported is 0.7850 0.0002g/mL. This
value indicates that the density is between 0.7850 and 0.7852g/mL.
2) Using the relative average deviation as an indication of precision, one would
report the density as 0.7850 g/mL with a relative average deviation of 0.3 units
for each 1000 units reported.
Relative average deviation( ppt)
=
Standard deviation: Statistics gives another common method of computing the quality
of experimental data or results called standard deviation. The standard deviation is
calculated by taking the sum of the deviations divided by one less than the number of
deviations and taking the square rood of the quotient. The standard deviation, σ, means
the likely hood of another measurement or result would have a 68% chance of falling in
the range of σ of the mean value. Using a range of two standard deviation units, 2σ,
then the next measurement or result should have a 99% chance of being with two
deviations from the mean.
1/2
d12 + d22 + ... + dn2
σ=
n-1
The size of σ is related to the precision of the results or data, thus a larger the value of σ
is less precise than a smaller value of σ.