st: generate a column of a summary statistic conditioning on the comparison of the values in two other columns

[David Peng <david.peng99@gmail.com>]

I have a dataset with four variables, A, B, C, and D. A is a variable
representing the customer number. B is the main variable of interest
(in this case the dollar amount of a customer order). C is the date a
customer order was placed and D is the date the same customer order
was delivered.

I would like to generate a column of summary statistic (let's say I
want the mean) in the table. Basically, for each customer order, I
would like to generate a mean value of the dollar amount for all of
the orders placed by a customer prior to the date the order is placed.

For each observation (i.e., a cusomer order) in the data table, I
would like to get:

bysort A: egen mean_dollar_amount=mean(B) if B is associated with a
delivery date D that is earlier than the order date C of the customer
order in question.


As an example, if I have an obervation representing an order placed by
the customer x with the order date of 12/30/2011, I would like to
generate the mean of the dollar amount for all of the orders that were
delivered earlier than the order date of 12/30/2011 for the order
mentioned above. I need a mean value like this for each observation
(i.e., customer order) in the data.

Thanks in advance for your help.
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