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From |
mochoa1@worldbank.org |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: data representation |

Date |
Fri, 27 Dec 2002 10:25:57 -0400 |

K.I. , I hope this is useful to get a solution for your problem. Type help datatypes to get the following info: 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, however, 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 help 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] 16.10 Precision and problems therein. Also see Manual: [U] 15 Data, [U] 16.10 Precision and problems therein, [R] data types On-line: help for compress, encode, format, recast My best regards, Marcelo * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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