Lots of times students complain that either their statistics
classes used silly examples that were too simple to ever be realistic, or that
their course was too complicated and thus they leave the class without the
capability of any practical application. A
recent study looking at the safety of GMO corn provides a great case study for
the practical application of the coefficient
of variation (CV).
In ‘Maternal and fetal
exposure to pesticides associated to genetically modified Foods in Eastern
Townships of Quebec, Canada’ the authors claim to have identified the toxin
Cry1Ab in the blood of pregnant women. Cry1Ab is a protein produced by the bacteria Bacillus thuringiensis (Bt) that is
toxic to certain insect pests. Cry1Ab
is just one version (event) of this Bt toxin.
Bt toxins have been used extensively by organic farmers and
biotechnology has enabled seed companies to develop corn plants that express
Cry1Ab proteins giving them a built in defense mechanism against insects
susceptible to the toxin, while preserving the biodiversity of friendly
insects. Bt genetics have also been
incorporated into cotton. The economic, environmental, safety, and health benefits
have made this a very popular tool used
by the majority of family farmers.
One of the major criticisms of the article was the use of
the test used to identify the Cry1Ab protein. In the article the authors state:
‘Cry1Ab protein levels
were determined in blood using a commercially available double antibody
sandwich(DAS)enzyme-linked immune sorbent assay.’
In previous research, the enzyme-linked immunosorbent assay or ELISA test has been shown to be one
of the most unreliable tests for detecting Cry1Ab proteins. Recall, the CV is relative measure of
variation measuring the standard deviation relative to the mean.
It can be used as a metric for risk and
reliability (such a consistent yield performance or stock returns). In the article
‘Comparison and Validation of Methods To Quantify Cry1Ab Toxin from Bacillus
thuringiensis for Standardization of Insect Bioassays’ the authors
investigate procedures commonly used to identify Cry1Ab. The authors explain:
“We compared three
methods of quantification on three different toxin preparations from
independent sources: enzyme-linked immunosorbent assay (ELISA), sodium dodecyl
sulfate-polyacrylamide gel electrophoresis and densitometry (SDS-PAGE/densitometry),
and the Bradford assay for total protein....The Bradford method resulted in
statistically higher estimates than either ELISA or SDSPAGE/ densitometry but
also provided the lowest coefficients of variation (CVs) for estimates of the
Cry1Ab concentration (from 2.4 to 5.4%). The CV of estimates obtained by ELISA
ranged from 12.8 to 26.5%, whereas the
CV of estimates obtained by SDS-PAGE/densitometry ranged from 0.2 to
15.4%....we conclude that standardization of Cry1Ab production and
quantification by SDS-PAGE/densitometry may improve data consistency.”
If we look at their reported statistics, we can see for ourselves
just how high the CV is on the ELISA test (and therefore how unreliable it is
as a method for quantifying Cry1Ab)
compared to other proven methods of quantification.
So there you have it. A practical example of an application
of a very basic statistic, the coefficient of variation.
References:
Maternal and fetal
exposure to pesticides associated to genetically modified foods in Eastern
Townships of Quebec, Canada. Reprod Toxicol. 2011 May;31(4):528-33. Epub
2011 Feb 18.
Aris A, Leblanc S.
Comparison and
Validation of Methods To Quantify Cry1Ab Toxin from Bacillus thuringiensis for
Standardization of Insect Bioassays. Andre´ L. B. Crespo,1 Terence A.
Spencer,1 Emily Nekl,2 Marianne Pusztai-Carey,3 William J. Moar,4 and Blair D.
Siegfried1* APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Jan. 2008, p. 130–135 Vol.
74, No. 1
A Meta-Analysis of Effects of Bt Cotton and Maize on
Nontarget Invertebrates. Michelle Marvier, Chanel McCreedy, James Regetz,
Peter Kareiva Science 8 June 2007: Vol. 316. no. 5830, pp. 1475 – 1477
Comparison of Fumonisin Concentrations in Kernels of
Transgenic Bt Maize Hybrids and Nontransgenic Hybrids. Munkvold, G.P. et al
. Plant Disease 83, 130-138 1999.
Indirect Reduction of Ear Molds and Associated
Mycotoxins in Bacillus thuringiensis Corn Under Controlled and Open Field
Conditions: Utility and Limitations. Dowd, J. Economic Entomology. 93
1669-1679 2000.
Impact of Bt
cotton on pesticide poisoning in smallholder agriculture: A panel data analysis.
Shahzad Kouser, Matin Qaim. Ecological Economics
Volume 70, Issue 11, 15 September 2011, Pages 2105–2113
Communal Benefits of Transgenic Corn. Bruce E.
Tabashnik Science 8 October 2010:Vol. 330. no. 6001, pp. 189 - 190DOI:
10.1126/science.1196864
