The most common cause of this error message is that you are trying to use a string variable with a command that only supports numeric variables. You can only tell the type of a variable by using the describe command.
This is easy to fix.
If you have a string variable and want to convert it to a numeric variable, you can use the encode command. If you have a string variable that has only numbers in it, then you can alternatively use the real() function.
. describe
Contains data
obs: 4
vars: 2
size: 48
------------------------------------------------------------------------
storage display value
variable name type format label variable label
------------------------------------------------------------------------
a str4 %9s
b str4 %9s
------------------------------------------------------------------------
Sorted by:
Note: dataset has changed since last saved
. list
+-------+
| a b |
|-------|
1. | 1 a |
2. | 2 b |
3. | 3 c |
4. | 4 d |
+-------+
. gen na = real(a)
. encode b, gen(nb)
. describe
Contains data
obs: 4
vars: 4
size: 80
------------------------------------------------------------------------
storage display value
variable name type format label variable label
------------------------------------------------------------------------
a str4 %9s
b str4 %9s
na float %9.0g
nb long %8.0g nb
------------------------------------------------------------------------
Sorted by:
Note: dataset has changed since last saved
. list
+-----------------+
| a b na nb |
|-----------------|
1. | 1 a 1 a |
2. | 2 b 2 b |
3. | 3 c 3 c |
4. | 4 d 4 d |
+-----------------+
Although nb is a numeric variable, it looks like a string variable because the encode command added value labels to it.
. list nb, nolab
+----+
| nb |
|----|
1. | 1 |
2. | 2 |
3. | 3 |
4. | 4 |
+----+
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