st: Generating mean by two groups

["Z. Aloha" <mai7777@gmail.com>]

Hi,
How can I generate a mean by two groups. That is I have different
location allocations (L)  and different dates (D), and I would like to
generate a new variable (M) that contains the mean of the price (P)
within each location on each date. An example is listed below:
P       L       D      M
1       A       1     1.5
2       A       1     1.5
3       B       1      3
3       B       1      3
5       A       2      5.5
6       A       2      5.5
3       B       2       5
7       B       2       5
2       A       3       3.5
5       A       3       3.5
4       B       3       6
8       B       3       6
12      A       4      8
4       A       4       8
6       B       4       4
2       B       4       4


Thanks for your help
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