Stata 15 help for data_types

[D] data types -- Quick reference for data types

Description

This entry provides a quick reference for data types allowed by Stata. See [U] 12 Data for details.

Remarks

Closest to Storage 0 without type Minimum Maximum being 0 bytes ---------------------------------------------------------------------- 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 ---------------------------------------------------------------------- Precision for float is 3.795x10^-8. Precision for double is 1.414x10^-16.

String storage Maximum type length Bytes ----------------------------------------- str1 1 1 str2 2 2 ... . . ... . . ... . . str2045 2045 2045 strL 2000000000 2000000000 -----------------------------------------

Each element of data is said to be either type numeric or type string. The word "real" is sometimes used in place of numeric. Associated with each data type is a storage type.

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.

Strings are stored as str#, for instance, str1, str2, str3, ..., str2045, or as strL. 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. A strL can hold strings of arbitrary lengths, up to 2000000000 characters, and can even hold binary data containing embedded \0 characters.

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. 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.


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