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HP 10bii Calculator - IRR/YR Calculations


The HP 10bii calculator determines IRR/YR for a set of cash flows using mathematical formulas that search for the answer. The process finds a solution by estimating an answer and then using that estimate to do another calculation. This is called an iterative process.
In most cases, the calculator finds the desired answer, since there is usually only one solution to the calculation. However, calculating IRR/YR for certain sets of cash flows is more complex. There may be more than one (or no) mathematical solution to the problem. In these cases, the calculator displays a message to help interpret what has happened.

Calculator symbol key

The procedures in this document use the following text to represent symbol keys:
Text Representation
colored key; shift key

Possible outcomes of calculating IRR/YR

These are the possible outcomes of an IRR/YR calculation:
  • Case 1.The calculator displays a positive answer. This is the only positive answer. However, one or more negative answers may exist.
  • Case 2. The calculator finds a negative answer but a single positive answer also exists. It displays "Pos IRR Also". To see the negative answer, press backspace to clear the message. To search for the positive answer, input a guess. There might also be additional negative answers.
  • Case 3. The calculator displays a negative answer and no message. This is the only answer.
  • Case 4. The calculator displays the message "Not Found". This indicates that the calculation is very complex. It might involve more than one positive or negative answer, or there may be no solution. To continue the calculation, store a guess.
  • Case 5. The calculator displays: "No Solution". There is no answer. This situation might be the result of an error, such as a mistake in keying in the cash flows. A common mistake that results in this message is putting the wrong sign on a cash flow. A valid cash-flow series for an IRR/YR calculation must have at least one positive and one negative cash flow.

Halting and restarting IRR/YR

The search for IRR/YR may take a relatively long time. The calculation can be halted at any time by pressing the C key. The message "Interrupted" is displayed. Pressing backspace now displays the current estimate for IRR/YR. The calculation can be resumed by:
  • Pressing SHIFT, STO, SHIFT, then IRR/YR while the current estimate is displayed in the calculator line. This continues the calculation from where it left off.
  • Storing a guess for IRR/YR.

Entering a guess for IRR/YR

To enter a guess, key in an estimate or IRR/YR and then press SHIFT, STO, SHIFT, then IRR/YR. A guess can be entered at these times:
  • Before beginning the calculation. A fairly accurate guess can reduce the time required to calculate an answer and reduce the chance of the calculator solving for an undesirable negative solution.
  • After the calculation has been interrupted.
  • After the calculator has halted the calculation due to any of the aforementioned cases. However, for cases 3 and 5, no other solutions will be found.
When calculating IRR/YR using a guess, the calculation halts when it finds an answer. However, there may be additional positive or negative answers, or no true solution at all. The search can be continued for another solution by halting the calculation and entering a different guess.One way to obtain a good guess for IRR/YR is to calculate the NPV for various interest rates. Since IRR/YR is the interest rate at which NPV equals zero, the best estimate of IRR/YR is the interest rate that yields the value for NPV closest to zero.






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