Hp12c Rounding up

[fperezl]

Hi, I just bought a Hp12c and for some reason when I do the following calculation I get a different rounding on other calculators. pv=-1 fv=2 i=6.5 pmt=0 ====> n= 11.0067 ----------> on HP17BII+ If I do the same problem on a HP12c = I get n= 12 anyone ? is there way to change the rounding on the hp12c ? if so ? how, why do we get different answers ? thanks

[David_M]

When solving for N, the 12c will always give an integer answer.  There's no method or setting that will change that.  Reference the HP12c User's Guide on pages 39-40.  I've included the text below for you:

Calculating the Number of Payments or Compounding Periods

1. Press [f] CLEAR [FIN] to clear the financial registers.
2. Enter the periodic interest rate, using [i] or [12÷].
3. Enter at least two of the following values:

• Present value, using [PV].
• Payment amount, using [PMT].
• Future value, using [FV].

(Note: Remember to observe the cash flow sign convention.)
4. If a PMT was entered, press g[BEG] or g[END] to set the payment mode.
5. Press [n] to calculate the number of payments or periods.

If the answer calculated is not an integer (that is, there would be nonzero digits to the right of the decimal point), the calculator rounds the answer up to the next higher integer before storing it in the n register and displaying it.* For example, if n were calculated as 318.15, 319.00 would be the displayed answer. n is rounded up by the calculator to show the total number of payments needed: n-1 equal, full payments, and one final, smaller payment. The calculator does not automatically adjust the values in the other financial registers to reflect n equal payments; rather, it allows you to choose which, if any, of the values to adjust.

* The calculator will round n down to the next lower integer if the fractional portion of n is less than 0.005.

[fperezl]

thanks for your response, 

Why would the HP17BII+ give a different number ?  when compared with other calculators and excel rounding gives a different number. 

Seems like a bug.

[David_M]

I have no direct knowledge of the exact circumstances that led to this kind of treatment by the original developers of the 12c (I'm just a user of hp calculators like you), but I do understand the basic premise.  Recall what "N" represents in the TVM calcuations: the number of periods/payments.  In the situation where the formulaic computation of N would result in a non-integer, that means there will be IP(N) equal payments, and a final payment of an amount less than the previous payments.  So the total number of payments is indeed the rounded up number, not some theoretical fractional amount.

Regardless of how "correct" that interpretation may be, it has caused confusion for enough people that most (perhaps all?) subsequent financial calculators from HP give fractional N results when solving for N.

It's also true that entering a fractional N as input (as opposed to solving for N) on a 12c has a special meaning (see the owner's manual on page 50 pertaining to Odd-Period Calculations).  That may also have been part of the decision-making for how N was to be treated on the 12c.

Hope this helps!

-David

[fperezl]

thanks for the response, it just makes me wonder. I get the idea about the decimals in the periods. BUt, not sure why other calculators will be more exact than the hp12c. Other hp calculators do provide the decimals, and it surprises me the platinum and the other newer editions (limited, etc) have not corrected this. While doing a number of exercises, this issue happened a number of times, and I couldn't find the answer in some "exams", anyway, just my 0.02 I appreciate your response to my previous email, thank you I just hope there was a way to make the hp12c show this. ( i think the feature should be included). thanks,

[cyrille]

Hello, The "other" more recent calculator give a mathematically correct answer. i.e: an answer that fulfill the mathematical equation. However, this answer does not really make sense in "real life". Kind of: What is the average's women number of babies: answer 1.8 This answer is the mathematical answer to the question. However, the "real world" answer to the question is Between "The average woman has 1 or 2 babies, but most likely 2 than 1". Same here for the N. You can not make 2.5 payments. You can make 2 payments, or 3 payments, or 2 payments with a third payment of a 1/2 value, but this is still 3 payments. So, the other calculators are not more exact, they are in fact giving you a false sense of exactitude by providing a non-sensical answer. However, this answer DOES provide you with additional information. In this case, an approximation of the size of your last payment (if the decimal value is large, last payment is a large % of normal payment)... But it is still a "wrong" answer. Or an answer to a different question that then one that you are asking. Why do modern calculators do it? Because it does provide more information and because it is/was assumed that the user would understand the subtlety of the answer. When such features were introduced, the few people that dealt with this did indeed understand it, especially those who went through the transition. However this is not the case anymore. Having asked the question to teachers, I have found out that very few of them do actually understand it! As for your other question. Why do the various 12C model not give the decimals? Mostly because the 12C is a "gold" standard and that HP can not change anything about the 12C without causing a an uproar. Cyrille

[BartdB]

Hi,

Further to Cyrille's answer and the practicality of the real world:

What bank is going to give a loan with 8.5 repayments? In fact short term loans are likely to be offered in 6 month intervals and house mortgages in 12 month intervals.

Or for an investment, let's say your mathematically correct calculator says it will take 65.5 months to reach your target value, the 12C will probably tell you it's 66 months. Which one is more correct?

The interest on the investment will probably be paid on a monthly basis. So at 65 months you will not have reached your target and at 66 months you will have slightly more than your target. But halfway between month 65 and 66? You will still only have month 65 's value because you are not going to get a partial interest payment halfway through the month. So the 12C is correct that you have to wait at least 66 months to reach the target value (you can then go back and calculate what you actual investment value would be at 66 months).

You say "it surprises me the platinum and the other newer editions (limited, etc) have not corrected this" when in actual fact the 12C approach is the practically correct way. So in my view there is nothing to "correct".

Note that the newer HP10bii+ has gone the way of providing the mathematically correct answer (i.e. fractional periods).

Best regards.

[paul0207]

I agree with what Cyrille and Bart have said. Nevertheless if you are still intered in getting a fractional N with the HP-12c please take a look at the following programs:

HP 12c Calculator - Actuarial Calculations

https://support.hp.com/us-en/document/bpia5043

Cheers,

Paul

[Maké]

Hi!, @Paul0207 :

You can, see ... 

https://support.hp.com/us-en/document/bpia5185

http://www.tvmcalcs.com/index.php/calculators/hp12c/hp12c_page1 (pages 1 to 3).

[fperezl]

i thank you for your feedback and help. From what you have answered "correct or not correct" 

You say "it surprises me the platinum and the other newer editions (limited, etc) have not corrected this" when in actual fact the 12C approach is the practically correct way. So in my view there is nothing to "correct".

I understand your point, however, i differ and therefore, don't agree with you in terms of whats correct or not correct. I n the end is about precision. Although, it would be difficult and perhaps unrelastic 12.X periods in the end i rather see that number than 13. 

I have seen the responses from other people and I see their point of view as well, in the end, if i want the decimals and "12.x" i would go with excel or any other spreadsheet. I am not looking for the interpretation here, thank you.