I want to calculate the mean and standard error of the mean for the amount spent per person visiting a store. Most people don't buy anything, so the distribution looks like this
${\rm P}({\rm purchased},{\rm spend}) = {\rm P}({\rm purchased}) {\space} {\rm P}({\rm spend} | {\rm purchased})$
where purchased and spend are two dependent variables -- dependent in that if purchased = False, then spend is necessarily 0.
One way to do this would be by taking the mean and standard error from the collection of amount spent by each person:
$ T = [\$13, \$6, \$0, \$0, \$0, \$0, \$0, \$0, \$0, \$5, \$0, \$0, \$0, ...]$
$ E(T) = \frac{1}{N} \sum_{i=1}^{N} T_{i}$
$ {\rm standard \space error} = \frac{\sqrt{\frac{1}{N-1} \sum_{i=1}^{N} (T_{i} - E(T))^2}}{\sqrt{N}} $
However, I'm worried about missing the inherent binomial distribution, given that some people bought something and others didn't.
Is there a more correct way to calculate the standard error of the mean?