MEd ACADEMY
04 Sept -08 Sep 2023
TUTORIAL 2
1. The reproducibility of a method for the determination of selenium in foods was
investigated by taking nine samples from a single batch of brown rice and determining
the selenium concentration in each. The following results were obtained:
0.07 0.07 0.08 0.07 0.07 0.08 0.09 µg g-1
Calculate the mean, standard deviation, relative standard deviation and 99% confidence
limits for the true concentration of these results. (Assume that there are no systematic
errors.)
2. The following values were obtained for the nitrile concentration (mg L-1) in a sample of
river water:
0.403 0.410 0.401 0.380
The last measurement is noticeably lower than the others and is thus a suspect:
Should it be rejected?
3. Consider the following set of replicate measurements and use it to answer all the
questions a-d:
data: 0.514, 0.503, 0.486, 0.497, 0.472
a. Calculate the (i) mean, (ii) median, (iii) range, (iv) standard deviation, (v) coefficient
of variation, (vi) 95% confidence interval for the data
b. The accepted value for the data set is 0.525. From the mean of this set, calculate (i)
the absolute error and (ii) the relative error in ppt (parts per thousand),
c. Calculate the 95% confidence interval for data set if s is a good estimate of σ, and has
a value of 0.015.
d. The last result in the data set may be an outlier. Apply the Q test (95% confidence
level) to determine whether or not there is a statistical basis for rejection.