  This paper introduces a new four-parameter lifetime model called the Weibull
Birnbaum-Saunders distribution. This new distribution represents a more
flexible model for the lifetime data. Its failure rate function can be
increasing, decreasing, upside-down bathtub shaped, bathtub-shaped or modified
bathtub shaped depending on its parameters. Some structural properties of the
proposed model are investigated including expansions for the cumulative and
density functions, moments, generating function, mean deviations, order
statistics and reliability. The maximum likelihood estimation method is used to
estimate the model parameters and the observed information matrix is
determined. The flexibility of the new model is shown by means of two real data
sets.
