. list
+---------------+
| i t x1 |
|---------------|
1. | a 1990 1 |
2. | a 1991 2 |
3. | b 1990 3 |
4. | b 1991 4 |
+---------------+
. table i,c(mean x1 sd x1)
----------------------------------
i | mean(x1) sd(x1)
----------+-----------------------
a | 1.5 .7071068
b | 3.5 .7071068
----------------------------------