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Re: st: Rounding Errors Stata 12


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Rounding Errors Stata 12
Date   Wed, 13 Feb 2013 14:56:52 +0000

Stas' view is not universal.

I don't know how big your numbers go, but using -double- can only
lessen the apparent problem, not fix it.

I'd predict from what you have told us that holding such data as
-float- will be entirely unproblematic
except for the one strange detail that you started with, that
occasionally such numbers will be displayed with
what look like spurious decimal places.

Nick

On Wed, Feb 13, 2013 at 2:47 PM, Gray, Charles <gray.c@east.ei.com> wrote:
> Yeah I've read about data types and storage. Here is what the Stata help
> window says:
>
> "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 float()."
>
> I guess I just don't follow why .8 is stored as .801. My concern with
> storing the variable as a double is memory usage. I've got several other
> datasets that I'll be appending onto this one and I'm just not sure I'll
> be able to store the variable as a double once I've appended all of the
> datasets onto each other.
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Stas
> Kolenikov
> Sent: Wednesday, February 13, 2013 9:38 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: Rounding Errors Stata 12
>
> Read about -help data types- and -insheet- your data as -double-s. The
> default -float- does not have enough accuracy. (Add it to the wish list
> for Stata 13: make -double- the default type, or get rid of -float-s
> whatsoever; I have -set type double- in my profile.do, and only generate
> -float- variables when I need to reproduce somebody's round-off errors.)
>
> --
> -- Stas Kolenikov, PhD, PStat (SSC)  ::  http://stas.kolenikov.name
> -- Senior Survey Statistician, Abt SRBI  ::  work email kolenikovs at
> srbi dot com
> -- Opinions stated in this email are mine only, and do not reflect the
> position of my employer
>
>
> On Wed, Feb 13, 2013 at 8:25 AM, Gray, Charles <gray.c@east.ei.com>
> wrote:
>> I am having an issue with Stata 12 adding decimal places to data that
> I
>> insheet. I simply have a dataset in .csv format. The dataset contains
> a
>> variable 'item_revenue.' When I open the dataset in Excel, several
>> observations have a value of 60237.8 for the 'item_revenue' variable.
>> However, when I insheet the dataset into Stata, these values change to
>> 60237.801. My insheet command is simply,
>>
>> insheet using "data.csv", comma clear
>>
>> My understanding is that the .csv format saves only the text and
> values
>> as they are displayed in cells of the active worksheet. So does anyone
>> know why Stata would add decimal places to a variable?
>>
>> Thanks,
>>
>> Charlie
>>
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