I've got myself confused with TVM results when using my HP19bii+ v my HP12CP and non-interger "n" (my wife just says, "why have two calculators" - which is a reasonable point however my secret is that I have three (also a HP10b - which gives the same results as the HP17bii+).

For instance if I run a scenario of:

P/YR = 1

n = 2.5

iP/YR = 10% (those were the days!)

PV = 100

PMT = 10

END MODE

HP17 bii+ FV = -153.81

This seems quite reasonable as it is approximately the same as replicating the inputs for 30 months (slight compounding difference I can cope with that).

If I run the same inputs for my HP12CP I get FV = -148.05

Now, reading the manual, I thought this was because the 12CP would take the "0.5" of "n" and consider it as half a month, ie 15 days. But if I try to model a 15 day period (ie P/YR = 24 and PMT = 10/24) followed by a two year period I get FV = -142.99 (which feels about right, but is a country mile away from the 12CP).

Can anyone shed a little light on this for me.

Thanks in anticipation.

When you enter a non-integer value for n in the 12c it enters odd-period mode and doesn't assume a payment until the first full period. In your example using 1 payment per year the 12c gives you 1/2 of a year interest, then one-year interest plus a payment, and finally another year's interest plus a payment.

The calculation looks like this

Beg End

Bal Int Pmt Bal

100.00 5.00 0.00 105.00

105.00 10.50 10.00 125.50

125.50 12.55 10.00 148.05

The 17bii doesn't do odd periods like that. When you key in 2.5 periods, it gives you 2.5 payments. You get an additional $5 at the end of the partial period that you don't get with the 12c.

edit: Sorry, I don't do tables well. The column headings should be Beginning Balance, Interest, Payment, and Ending Balance

*Edited: 17 Sept 2011, 9:46 a.m. *

Beg End

Bal Int Pmt Bal

100.00 5.00 0.00 105.00

105.00 10.50 10.00 125.50

125.50 12.55 10.00 148.05

Brilliant - I had tried putting the half-period at the end (after all the HP12 was set to "end"....) but with your helpful table I can now clearly see what the result means - mystry solved. Many thanks.

Paul