Stata 11 help for data_types
help data types
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Title
[D] data types -- Quick reference for data types
Description
This entry provides a quick reference for datatypes allowed by Stata.
Remarks
Closest to
Storage 0 without
type Minimum Maximum being 0 bytes
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byte -127 100 +/-1 1
int -32,767 32,740 +/-1 2
long -2,147,483,647 2,147,483,620 +/-1 4
float -1.70141173319*10^38 1.70141173319*10^38 +/-10^-38 4
double -8.9884656743*10^307 8.9884656743*10^307 +/-10^-323 8
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Precision for float is 3.795x10^-8.
Precision for double is 1.414x10^-16.
String
storage Maximum
type length Bytes
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str1 1 1
str2 2 2
... . .
... . .
... . .
str244 244 244
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Each element of data is said to be either type string or numeric. The
word real is sometimes used in place of numeric. Associated with each
data type is a storage type.
Strings are stored as str#, for instance, str1, str2, str3, ..., str244.
The number after the str indicates the maximum length of the string. A
str5 could hold the word "male", but not the word "female" because
"female" has six characters.
Numbers are stored as byte, int, long, float, or double, with the default
being float. byte, int, and long are said to be of "integer" type in
that they can hold only integers.
Stata keeps data in memory, and you should record your data as
parsimoniously as possible. If you have a string variable that has
maximum length 6, it would waste memory to store it as a str20.
Similarly, if you have an integer variable, it would be a waste to store
it as a double.
Precision of numeric storage types
floats have about 7 digits of accuracy; the magnitude of the number does
not matter. Thus, 1234567 can be stored perfectly as a float, as can
1234567e+20. The number 123456789, however, would be rounded to
123456792. In general, this rounding does not matter.
If you are storing identification numbers, the rounding could matter. If
the identification numbers are integers and take 9 digits or less, store
them as longs; otherwise, store them as doubles. doubles have 16 digits
of accuracy.
Stata stores numbers in binary, and this has a second effect on numbers
less than 1. 1/10 has no perfect binary representation just as 1/11 has
no perfect decimal representation. In float, .1 is stored as
.10000000149011612. Note that there are 7 digits of accuracy, just as
with numbers larger than 1. Stata, however, performs all calculations in
double precision. If you were to store 0.1 in a float called x and then
ask, say, "list if x==.1", there would be nothing in the list. The .1
that you just typed was converted to double, with 16 digits of accuracy
(.100000000000000014...), and that number is never equal to 0.1 stored
with float accuracy.
One solution is to type "list if x==float(.1)". The float() function
rounds its argument to float accuracy; see [D] functions. The other
alternative would be store your data as double, but this is probably a
waste of memory. Few people have data that is accurate to 1 part in 10
to the 7th. Among the exceptions are banks, who keep records accurate to
the penny on amounts of billions of dollars. If you are dealing with
such financial data, store your dollar amounts as doubles. See [U] 13.11
Precision and problems therein.
Also see
Manual: [D] data types
Help: [D] compress, [D] destring, [D] encode, [D] format, [D] recast
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