How to assign mean values of groups to missing observations?

[NonSleeper]

The input data looks like below: 

State	County	HH	Var
1	1	1	1
1	1	2	2
1	1	3
1	2	1	4
1	2	2	5
1	3	1	6
2	1	1	1
2	1	2	2
2	2	1	3
2	3	1	4
2	3	2

The set of 3 variables state, county, and households (HH) specifies unique codes for observations.

The variable Var is, say, a characteristic of households and it has missing data. My question is: How can I replace those missing observations with the mean values of households of the same county?

The output data should look like:

State	County	HH	Var
1	1	1	1
1	1	2	2
1	1	3	1.5
1	2	1	4
1	2	2	5
1	3	1	6
2	1	1	1
2	1	2	2
2	2	1	3
2	3	1	4
2	3	2	4

[NonSleeper]

After some search, it seems quite straightforward with PROC STANDARD. Sorry for bringing this up, but for those who are as novice:

proc standard data=have replace;

var Var;

by state county;

run;

[LinusH]

Thanks for sharing your solution!