Statistics chm 235

CHM 235 – Dr. Skrabal

Statistics for Quantitative Analysis
• Statistics: Set of mathematical tools used to describe
and make judgments about data
• Type of statistics we will talk about in this class has
important assumption associated with it:
Experimental variation in the population from which samples
are drawn has a normal (Gaussian, bell-shaped) distribution.
- Parametric vs. non-parametric statistics

Normal distribution
• Infinite members of group:
population

• Characterize population by taking
samples
• The larger the number of samples,
the closer the distribution becomes to
normal
• Equation of normal distribution:

1
− ( x − µ ) 2 / 2σ 2
y=
e
σ 2π

Normal distribution
• Estimate of mean value
of population = µ
• Estimate of mean value
of samples = x

Mean =

x

=

∑x

i

i

n

Normal distribution
•

Degree of scatter (measure of
central tendency) of population is
quantified by calculating the
standard deviation

•

Std. dev. of population = σ

•

Std. dev. of sample = s

s=

∑ ( xi − x ) 2
i

n −1

x±s
•

Characterize sample by calculating

Standard deviation and the
normal distribution
• Standard deviation
defines the shape of the
normal distribution
(particularly width)
• Larger std. dev. means
more scatter about the
mean, worse precision.
• Smaller std. dev. means
less scatter about the
mean, better precision.

Standard deviation and the
normal distribution
• There is a well-defined relationship
between the std. dev. of a population
and the normal distribution of the
population:
∀ µ ± 1σ encompasses 68.3 % of
measurements
∀ µ ± 2σ encompasses 95.5% of
measurements
∀ µ ± 3σ encompasses 99.7% of
measurements
• (May also consider these
percentages of area under the curve)

Example of mean and standard
deviation calculation
Consider Cu data: 5.23, 5.79, 6.21, 5.88, 6.02 nM

x

= 5.826 nM  5.82 nM

s = 0.368 nM  0.36 nM
Answer: 5.8 2 ± 0.3 6 nM or 5.8 ± 0.4 nM
Learn how to use the statistical functions on your
calculator. Do this example by longhand calculation
once, and also by calculator to verify that you’ll get
exactly the same answer. Then use your calculator
for all future calculations.

Relative standard deviation (rsd)
or coefficient of variation (CV)
s
rsd or CV =  100
x

From previous example,
rsd = (0.36 nM/5.82 nM) 100 = 6. 1 % or 6%

Standard error
•

•

•

Tells us that standard deviation of set of samples should
decrease if we take more measurements
Standard error =

sx

=

s
n

Take twice as many measurements, s decreases by

2 ≈ 1.4

4 =2

•

Take 4x as many measurements, s decreases by

•

There are several quantitative ways to determine the sample
size required to achieve a desired precision for various statistical
applications. Can consult statistics textbooks for further
information; e.g. J.H. Zar, Biostatistical Analysis

Variance
Used in many other statistical calculations and tests
Variance = s2
From previous example, s = 0.36
s2 = (0.36)2 = 0. 129 (not rounded because it is usually
used in further calculations)

Average deviation
•

Another way to express
degree of scatter or
uncertainty in data. Not as
statistically meaningful as
standard deviation, but
useful for small samples.

d=

∑( x − x )
i

i

n

Using previous data:

d=

5.23 − 5.82 + 5.79 − 5.82 + 6.21 − 5.82 + 5.88 − 5.82 + 6.02 − 5.82
5

d = 0.25 → 0.25 or 0.2 nM

Answer : 5.82 ± 0.25 nM or 5.8 ± 0.2 nM

Relative average deviation (RAD)
d 
RAD =  100 (as percentage)
x
 
d 
RAD =  1000 (as parts per thousand , ppt )
x
 
Using previous data,
RAD = (0. 25/5.82) 100 = 4. 2 or 4%
RAD = (0. 25/5.82) 1000 = 42 ppt
 4. 2 x 10 1 or 4 x 10 1 ppt ( 0 / 00 )

Some useful statistical tests
• To characterize or make judgments about data
• Tests that use the Student’s t distribution
– Confidence intervals
– Comparing a measured result with a “known”
value
– Comparing replicate measurements (comparison
of means of two sets of data)

From D.C. Harris (2003) Quantitative Chemical Analysis, 6th Ed.

Confidence intervals
• Quantifies how far the true mean (µ) lies from the
measured mean, x. Uses the mean and standard
deviation of the sample.

µ=x±

ts
n

where t is from the t-table and n = number of
measurements.
Degrees of freedom (df) = n - 1 for the CI.

Example of calculating a
confidence interval
Consider measurement of dissolved Ti
in a standard seawater (NASS-3):
Data: 1.34, 1.15, 1.28, 1.18, 1.33, 1.65,
1.48 nM
DF = n – 1 = 7 – 1 = 6

x = 1.34 nM or 1.3 nM

s = 0.17 or 0.2 nM
95% confidence interval

t(df=6,95%) = 2.447
CI 95 = 1.3 ± 0.16 or 1.3 ± 0.2 nM
50% confidence interval

t(df=6,50%) = 0.718
CI 50 = 1.3 ± 0.05 nM

µ=x±

ts
n

Interpreting the confidence interval
• For a 95% CI, there is a 95% probability that the true
mean (µ) lies between the range 1.3 ± 0.2 nM, or
between 1.1 and 1.5 nM
• For a 50% CI, there is a 50% probability that the true
mean lies between the range 1.3 ± 0.05 nM, or
between 1.25 and 1.35 nM
• Note that CI will decrease as n is increased
• Useful for characterizing data that are regularly
obtained; e.g., quality assurance, quality control

Comparing a measured result
with a “known” value
• “Known” value would typically be a certified value
from a standard reference material (SRM)
• Another application of the t statistic

t calc =

known value − x
s

n

Will compare tcalc to tabulated value of t at appropriate df
and CL.
df = n -1 for this test

Comparing a measured result
with a “known” value--example
Dissolved Fe analysis verified using NASS-3 seawater SRM
Certified value = 5.85 nM
Experimental results: 5.76 ± 0.17 nM (n = 10)
tcalc =

known value − x
s

n

=

5.85 − 5.7 6
0.17

10

= 1.674

(Keep 3 decimal places for comparison to table.)
Compare to ttable; df = 10 - 1 = 9, 95% CL

ttable(df=9,95% CL) = 2.262
If |tcalc| < ttable, results are not significantly different at the 95% CL.
If |tcalc| ≥ ttable, results are significantly different at the 95% CL.
For this example, tcalc < ttest, so experimental results are not significantly
different at the 95% CL

Comparing replicate measurements or
comparing means of two sets of data
• Yet another application of the t statistic
• Example: Given the same sample analyzed by two
different methods, do the two methods give the “same”
result?

t calc =
s pooled =

x1 − x 2

n1 n2

s pooled

n1 + n2

2
s12 (n1 −1) + s 2 (n2 −1)
n1 + n2 − 2

Will compare tcalc to tabulated value of t at appropriate df
and CL.
df = n1 + n2 – 2 for this test

Comparing replicate measurements
or comparing means of two sets of
data—example
Determination of nickel in sewage
sludge
using two
Method 1: Atomic absorption different methods
Method 2: Spectrophotometry
spectroscopy
Data: 3.91, 4.02, 3.86, 3.99 mg/g

Data: 3.52, 3.77, 3.49, 3.59 mg/g
= 3.59 mg/g

x1 = 3.945 mg/g

x2

s1 = 0.073 mg/g

s2 = 0.12 mg/g

n1 = 4

n2 = 4

comparing means of two sets of data—
example
s pooled =

tcalc =

2
s12 (n1 −1) + s2 (n2 −1)
n1 + n2 − 2

x1 − x2

n1 n2

s pooled

n1 + n2

=

=

(0.07 3 ) 2 (4 −1) + (0.12 ) 2 (4 −1)
4+4−2

3.945 − 3.59
0.0993

(4)(4)
4+4

= 0.0993

= 5.056

Note: Keep 3 decimal places to compare to ttable.
Compare to ttable at df = 4 + 4 – 2 = 6 and 95% CL.
ttable(df=6,95% CL) = 2.447
If |tcalc| < ttable, results are not significantly different at the 95%. CL.
If |tcalc| ≥ ttable, results are significantly different at the 95% CL.
Since |tcalc| (5.056) ≥ ttable (2.447), results from the two methods are
significantly different at the 95% CL.

comparing means of two sets of data
Wait a minute! There is an important assumption
associated with this t-test:
It is assumed that the standard deviations (i.e., the
precision) of the two sets of data being compared are not
significantly different.
• How do you test to see if the two std. devs. are different?
• How do you compare two sets of data whose std. devs.
are significantly different?

F-test to compare standard
deviations
• Used to determine if std. devs. are significantly
different before application of t-test to compare
replicate measurements or compare means of two
sets of data
• Also used as a simple general test to compare the
precision (as measured by the std. devs.) of two sets
of data
• Uses F distribution

F-test to compare standard
deviations
Will compute Fcalc and compare to Ftable.

Fcalc

=

s12
2
s2

where s1 > s2

DF = n1 - 1 and n2 - 1 for this test.
Choose confidence level (95% is a typical CL).

From D.C. Harris (2003) Quantitative Chemical Analysis, 6th Ed

.

F-test to compare standard deviations
From previous example:
Let s1 = 0.12 and s2 = 0.073

Fcalc

=

s12
2
s2

=

(0.12 ) 2
(0.07 3 ) 2

= 2.70

Note: Keep 2 or 3 decimal places to compare with F table.
Compare Fcalc to Ftable at df = (n1 -1, n2 -1) = 3,3 and 95% CL.
If Fcalc < Ftable, std. devs. are not significantly different at 95% CL.
If Fcalc ≥ Ftable, std. devs. are significantly different at 95% CL.
Ftable(df=3,3;95% CL) = 9.28
Since Fcalc (2.70) < Ftable (9.28), std. devs. of the two sets of data are
not significantly different at the 95% CL. (Precisions are
similar.)

comparing means of two sets of data-revisited
The use of the t-test for comparing means was justified
for the previous example because we showed that
standard deviations of the two sets of data were not
significantly different.
If the F-test shows that std. devs. of two sets of data
are significantly different and you need to compare
the means, use a different version of the t-test 

comparing means from two sets of data
when std. devs. are significantly
different

x1 − x2

tcalc

=

DF





2
2
2
( s1 / n1 + s2 / n2 )


=  2
−2
2
 ( s1 / n1 ) 2
( s2 / n2 ) 2  


+
 n +1
 1
n2 + 1  



2
s12 / n1 + s2 / n2

Flowchart for comparing means of two
sets of data or replicate
measurements
Use F-test to see if std.
devs. of the 2 sets of
data are significantly
different or not

Std. devs. are
significantly different
Use the 2nd version
of the t-test (the
beastly version)

Std. devs. are not
significantly different
Use the 1st version of the
t-test (see previous, fully
worked-out example)

One last comment on the F-test
Note that the F-test can be used to simply test whether
or not two sets of data have statistically similar
precisions or not.
Can use to answer a question such as: Do method one
and method two provide similar precisions for the
analysis of the same analyte?

Evaluating questionable data
points using the Q-test
• Need a way to test questionable data points (outliers) in an
unbiased way.
• Q-test is a common method to do this.
• Requires 4 or more data points to apply.
Calculate Qcalc and compare to Qtable
Qcalc = gap/range
Gap = (difference between questionable data pt. and its
nearest neighbor)
Range = (largest data point – smallest data point)

Evaluating questionable data
points using the Q-test--example
Consider set of data; Cu values in sewage sample:
9.52, 10.7, 13.1, 9.71, 10.3, 9.99 mg/L
Arrange data in increasing or decreasing order:
9.52, 9.71, 9.99, 10.3, 10.7, 13.1
The questionable data point (outlier) is 13.1
Calculate

Qcalc

=

gap
range

=

(13.1 − 10.7)
(13.1 − 9.52)

= 0.670

Compare Qcalc to Qtable for n observations and desired CL (90% or
95% is typical). It is desirable to keep 2-3 decimal places in Q calc
so judgment from table can be made.
Qtable (n=6,90% CL) = 0.56

From G.D. Christian (1994) Analytical Chemistry, 5th Ed.

Evaluating questionable data points
using the Q-test--example
If Qcalc < Qtable, do not reject questionable data point at stated CL.
If Qcalc ≥ Qtable, reject questionable data point at stated CL.
From previous example,
Qcalc (0.670) > Qtable (0.56), so reject data point at 90% CL.
Subsequent calculations (e.g., mean and standard deviation)
should then exclude the rejected point.
Mean and std. dev. of remaining data: 10.04 ± 0.47 mg/L

Statistics chm 235

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