Why does the command complain that there are no observations?
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Title
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Encoding a string variable
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Author
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James Hardin, StataCorp
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Date
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January 1996; minor revision March 2001
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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 (99.9% of memory free)
------------------------------------------------------------------------
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 (99.9% of memory free)
------------------------------------------------------------------------
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 |
+----+
Warning:
If you have more than 67,784 unique values of the string variables that you are encoding,
encode will complain.
If that is the case, then you can use
. egen nb = group(b)
which will generate a numeric variable nb that does not have
value labels.
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