Biology Reference
In-Depth Information
FIGURE 6.1
Graphical representation of the problem to
be solved by PCA. (A) Scatter plot of individuals scored on
two traits,
X
1
and
X
2
; (B) an ellipse enclosing the scatter of
points shown in part (A); (C) a line through the scatter and
the perpendicular distances of the individuals from that
axis. The goal of PCA is to find the line that minimizes the
sum of those squared distances.
(A)
X
1
(B)
X
1
(C)
X
1
vertically onto a sheet of wax paper, then repeating the process with the other half of
the watermelon and the other side of the paper. The result of this mapping is a two-
dimensional elliptical distribution similar to the first example. This ellipse represents the
variance that is not described by PC1. Thus, the next step of the three-dimensional PCA is
