st: Creating a Variable for the average by subgroup for a panel dataset

[gianluca manzi <gianluca.manzi1992@gmail.com>]

Dear Statalist,

How can I calculate the average of a variable by the subgroup of
several other variables?

I have a panel dataset of individuals incomes, the state they live in,
their ages and genders.  I want to calculate the average income for
individual i at time period t based on these characteristics and
create a variable for this (to use as a "reference income" individual
i in time t will compare his income to).

So (say) I want to create a variable that will calculate the average
income for all women aged 20-30 who live in California;  the average
income for all men aged 20-30 who live in California;  the average
income for all women aged 30-40 who live in California;  the average
income for all men aged 30-40 who live in California;  the average
income for all women aged 20-30 who live in Texas etc etc.

The purpose of this is to create a "reference income" which individual
i in period t will compare his/her income to. So if the individual is
woman, aged 22, who lives in California, she'll compare her income to
the average income of 20-30 year old women who live in California. How
do I create this variable?

Thank you!
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