  The duration distribution of 408 GRBs with measured both duration $T_{90}$
and redshift $z$ is examined. Mixtures of a number of distributions (standard
normal, skew-normal, sinh-arcsinh, and alpha-skew-normal) are fitted to the
observed and intrinsic durations using the maximum log-likelihood method. The
best fit is chosen via the Akaike information critetion. The aim of this work
is to assess the presence of the presumed intermediate GRB class, and to
provide a phenomenological model more appropriate than the common mixture of
standard Gaussians. While $\log T^{obs}_{90}$ are well described by a truly
trimodal fit, after moving to the rest frame the statistically most significant
fit is unimodal. To trace the source of this discrepancy, 334 GRBs observed
only by $Swift$/BAT are examined in the same way. In the observer frame, this
results in a number of statistically plausible descriptions, being uni- and
bimodal, and with the number of components ranging from one to three. After
moving to the rest frame, no unambiguous conclusions may be put forward. It is
concluded that the size of the sample is not big enough to infer reliably GRB
properties based on a univariate statistical reasoning only.
