  A multivariate analysis of gamma-ray burst (GRB) bulk properties is presented
to discriminate between distinct classes of GRBs. Several variables
representing burst duration, fluence and spectral hardness are considered. Two
multivariate clustering procedures are used on a sample of 797 bursts from the
Third BATSE Catalog: a nonparametric average linkage hierarchical agglomerative
clustering procedure validated with Wilks' $\Lambda^*$ and other MANOVA tests;
and a parametric maximum likelihood model-based clustering procedure assuming
multinormal populations calculated with the EM Algorithm and validated with the
Bayesian Information Criterion.
  The two methods yield very similar results. The BATSE GRB population consists
of three classes with the following Duration/Fluence/Spectrum bulk properties:
Class I with long/bright/intermediate bursts, Class II with short/hard/faint
bursts, and Class III with intermediate/intermediate/soft bursts. One outlier
with poor data is also present. Classes I and II correspond to those reported
by Kouveliotou et al. (1993), but Class III is clearly defined here for the
first time.
