| LDFA is a multivariate
technique used in special applications where there are
several intact groups (random assignment may be
impossible) and they have been measured on several
independent measures. Thus, you will want to describe how
these groups differ on the basis of these measures. In
this case, classification and prediction is the main
objective. (When there are only two groups involved, a
similar procedure known as logistic regression is
preffered.) This
could be done by conducting univariate F tests for
each independent variable across the intact groups. As it
is similar to already discussed techniques, this method
is redundant. Instead, it is better to construct linear
combinations of the original variables which eliminate
intercorrelations between variables. This composite
function will best discriminate between the groups on a
multidimensional basis.
Note that LDFA is, in many
ways, similar to Multiple Regression. More importantly,
note that the purposes and techniques of LDFA are very
similar mathematically to MANOVA (except that the roles
of x and y are interchanged). The composite
is determined in the same way, such that the groups
maximally differ. The maximum number of dimensions which
can be calculated is the smaller value of the following
two: (a) the number of groups minus one, or (b) the
number of continuous variables. And like before, each
composite is formed from the residual of the previous,
thereby making each orthogonal.
Like MANOVA, LDFA is
tested for significance by the use of Wilk's Lambda.
Follow-up analyses are the same as those for MANOVA (see
the previous page). An analysis of the significant
findings of LDFA can give a Confusion Matrix, a
table showing how accurate group membership is predicted
by the composite function(s). Each "axis" is
labeled by the groups: one axis being the actual group
membership, and the other being the group membership
predicted by the composite function; the percentages
indicated on the diagonal are "hits"--correct
predictions- and the percentages that are not on the
diagonal are "misses"--incorrect predictions or
errors.
| |
REAL
GROUP MEMBERSHIP |
| |
G1 |
G2 |
G3 |
G4 |
G5 |
| G1 |
Hit |
Miss |
Miss |
Miss |
Miss |
| G2 |
Miss |
Hit |
Miss |
Miss |
Miss |
| G3 |
Miss |
Miss |
Hit |
Miss |
Miss |
| G4 |
Miss |
Miss |
Miss |
Hit |
Miss |
| G5 |
Miss |
Miss |
Miss |
Miss |
Hit |
|