

   JJaannkkaa HHaarrddnneessss ddaattaa

   AArrgguummeennttss::

       Dens: the air-dry timber density for the tree, in pound
             per cubic foot

       Hard: the corresponding Janka hardness, in Janka units.

   DDeettaaiillss::

        A low-degree polynomial regression may be an appropri-
        ate model.  A log or square-root transformation of
        hardness may also be useful.

   SSUUMMMMAARRYY::

        Janka hardness is an important structural property of
        Australian timbers, but is difficult to measure
        directly.  It is, however, directly related to the den-
        sity of the timber which is relatively easy to measure.
        In an effort to develop a method of predicting Janka
        hardness from density, the Janka hardness (here consid-
        ered the response) and the density of 36 Australian
        eucalypt hardwoods was measured.

   DDAATTAA DDEESSCCRRIIPPTTIIOONN::

        The data frame `janka' has two components:

   SSOOUURRCCEE::

        E. J. Williams (1959) Regression Analysis.  Wiley, New
        York. p. 43.

