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*Subject*: **Re: [office-formula] tinv and friends**

*From*:**"Andreas J. Guelzow" <aguelzow@math.concordia.ab.ca>***To*: "Eike Rathke" <erack@sun.com>, "office-formula@lists.oasis-open.org" <office-formula@lists.oasis-open.org>*Date*: Fri, 20 Jul 2007 09:46:03 -0600

On 7/20/2007, "Eike Rathke" <erack@sun.com> wrote: >Hi Andreas, > >On Thursday, 2007-07-19 14:31:07 -0600, Andreas J. Guelzow wrote: > >> In the current draft functions such as TINV require the degree of >> freedom to be an integer that is at least 1. Our don't see a >> mathematical reason for the "integer" requirement. > >Correct me if I'm wrong, but isn't degrees of freedom the number of >parameters that can vary? The "degrees of freedom" is a parameter for the Student T Distribution. In many places where the Student T Distribution is used this happens to be an integer. Occasionally an non-integer is being approximated by the next larger integer since caclulating the Student T Distribution is hard and so tables were used. > How much sense would one and a half parameter >make? I'll try to remember to dig out an example where non-integer df is used. (These are not your common text book exercises since they assume tables...) > >> Is there some other reason? > >The spreadsheet applications I checked (Calc, Gnumeric, Excel; Kspread >1.5.0 seems not to have it) deliver identical results for fractional >values as if truncated to an integer. True for Gnumeric 1.7.10 and before, not true for current cvs. (We had a bug report with respect to large degrees of freedom which caused this to become an issue.) If you give a mathematical program such as Mathematica a try you will see that they give you different values for non-integer df, in fact the Student T Distribution is continuous in df. I think that this is a situation where we have to decide whether to go with what spreadsheets do or with what is mathematically sensible. Andreas

**References**:**Re: [office-formula] tinv and friends***From:*Eike Rathke <erack@sun.com>

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