  The mean ground state occupation number and condensate fluctuations of
interacting and non-interacting Bose gases confined in a harmonic trap are
considered by using a canonical ensemble approach. To obtain the mean ground
state occupation number and the condensate fluctuations, an analytical
description for the probability distribution function of the condensate is
provided directly starting from the analysis of the partition function of the
system. For the ideal Bose gas, the probability distribution function is found
to be a Gaussian one for the case of the harmonic trap. For the interacting
Bose gas, using a unified approach the condensate fluctuations are calculated
based on the lowest-order perturbation method and on Bogoliubov theory. It is
found that the condensate fluctuations based on the lowest-order perturbation
theory follow the law $<delta^{2}N_{\bf 0}>\sim N$, while the fluctuations
based on Bogoliubov theory behave as $N^{4/3}$.
