Correlation analyzes of isoprenoid accumulation - a
statistical meta-analysis
All correlation analyses were performed with the aid of R version 3.3.0
(R Core Team, 2016, https://www.R-project.org/) using the outliers
(Komsta, 2011, R package version 0.14,
https://CRAN.R-project.org/package=outliers) and the gplots (Warnes et
al., 2016, R package version 3.0.1,
https://CRAN.R-project.org/package=gplots). The significance level α of
0.001 was assumed in all statistical tests.
Although for each accession the level of each metabolite was measured in
triplicate, the values thus obtained were analyzed separately, as
indicated by the number of experimental points in the respective figures
(which equals three times the number of accessions). Means were not
calculated, and this approach was employed deliberately to avoid the
problem of adjusting and weighing mean values and to allow testing for
outliers among single replicates instead of among mean values.
The Shapiro-Wilk test (Shapiro and Wilk 1965) was used to assess the
agreement of isoprenoid content in the populations with the Gaussian
distribution. Since, even after filtering out of extreme values with the
Grubbs’ test for outliers (Grubbs 1950), a vast majority of the
distributions were found non-Gaussian, further analyses were performed
using non-parametric methods. Consequently, a correlation matrix for the
seven investigated isoprenoids was calculated accordingly to the
Spearman’s rank correlation coefficients (Spearman 1904).
A hierarchical cluster analysis of the correlation matrix was performed
according to the Ward criterion (Ward 1963).