On Tue, Jun 9, 2009 at 12:51 AM, <[email protected]> wrote: > > --- On Mon, 6/8/09, Skipper Seabold <[email protected]> wrote: > >> I forgot the last payment (which doesn't earn any >> interest), so one more 100. > > So in fact they're not in agreement? > >> pretty soon. I don't have a more permanent reference >> for fv offhand, >> but it should be in any corporate finance text etc. >> Most of these >> type of "formulas" use basic results of geometric series to >> simplify. > > Let me be more specific about the difference between what we have and what > I'm finding in print. Essentially, it boils down to this: in every source > I've found, two "different" present/future values are discussed, that for a > single amount, and that for a constant (i.e., not even the first "payment" is > allowed to be different) periodic payment. I have not been able to find a > single printed reference that gives a formula for (or even discusses, for > that matter) the combination of these two, which is clearly what we have > implemented (and which is, just as clearly, actually seen in practice). > > Now, my lazy side simply hopes that my stridency will finally cause someone > to pipe up and say "look, dummy, it's in Schmoe, Joe, 2005. "Advanced > Financial Practice." Financial Press, NY NY. There's your reference; find > it and look it up if you don't trust me" and then I'll feel like we've at > least covered our communal rear-end. But my more conscientious side worries > that, if I've had so much trouble finding our more "advanced" definition (and > I have tried, believe me), then I'm concerned that what your typical student > (for example) is most likely to encounter is one of those simpler > definitions, and thus get confused (at best) if they look at our help doc and > find quite a different (at least superficially) definition (or worse, don't > look at the help doc, and either can't get the function to work because the > required number of inputs doesn't match what they're expecting from their > text, or somehow manage to get it to work, but get an answer very > different from that given in other sources, e.g., the answers in the back of > their text.) > > One obvious answer to this dilemma is to explain this discrepancy in the help > doc, but then we have to explain - clearly and lucidly, mind you - how one > uses our functions for the two simpler cases, how/why the formula we use is > the combination of the other two, etc. (it's rather hard to anticipate, for > me at least, all the possible confusions this discrepancy might create) and > in any event, somehow I don't really think something so necessarily elaborate > is appropriate in this case. So, again, given that fv and pv (and by > extension, nper, pmt, and rate) have multiple definitions floating around out > there, I sincerely think we should "punt" (my apologies to those unfamiliar > w/ the American "football" metaphor), i.e., rid ourselves of this nightmare, > esp. in light of what I feel are compelling, independent arguments against > the inclusion of these functions in this library in the first place. > > Sorry for my stridency, and thank you for your time and patience. >
I guess non of them found a textbook either Josef just some samples Note: Applications do not agree on the answer for IPMT(5%/12;10;360;10000;0;1), for payments at the beginning of each period. Kspread agrees with Gnumeric on the answer listed above. Excel and OOo both get -410.38 as the result. TODO: which is correct? Note: OpenOffice.org 2.0 and Excel returns different results to mot of the cases. The following use the result of Excel. Note: Gnumeric gives an error for negative rates. Excel and OOo2 do not. For NPER(-1%;-100;1000), OOo2 gives 9.48, Excel produces 9.483283066, Gnumeric gives a #DIV/0 error. This appears to be a bug in Gnumeric. _______________________________________________ Numpy-discussion mailing list [email protected] http://mail.scipy.org/mailman/listinfo/numpy-discussion
