The logarithmic transformation of the data is useful here for a number of
reasons, including correction for heteroscedasticity and allowing the
option of interpreting demand behavior in terms of either shares or points
bid (since shares=points/price). The one drawback is that for certain
firms in certain rounds IPF demand is zero, even though II demand is
nonzero for all available firms. In order to keep these firms in the
sample, we set the log of IPF demand to zero in these cases. <p>