We see that i. depends on the second partial derivative of the option price
with respect to the stock price. This dependence of iL on the curvature of the
option curve conforms with the geometrical representation of the hedge
return in the previous section.
Clearly, since S and T will change over time, the return on a hedge in
which the number of options held is kept constant will be heteroscedastic. If
there considerable variation, there will be problems in making statistical
inferences about hedge return.
Table 1 shcws how i depends on S and 7: Whilst iAt is clearly quite a
smooth function of S and 7; as might have been expected from examination
of its functional form above, there is nevertheless considerable variation over
the domain considered. In particular, option hedges that expire ‘close to the
money’ will exhibit a considerable amount of heteroscedasticity. This heteroscedasticity
could be removed, at least in principle, either by adjusting the
trading interval At inversely with i. or by adjusting the number of options
held inversely with i (whilst, of course, still holding an appropriate short
position in the underlying stock). Table 1 indicates that unless this is done there can be quite extreme heteroscedasticity in the hedge returns, particularly
in the last unit of time prior to expiration. (This example was
constructed with reasonable fiarameter values and with the time ,unit
corresponding to one quarter of a year and At corresponding to one trading
day.)