Talk:Documentation/Calc Functions/RRI

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    SF Comments

    (1) Summary. Suggest using the ODF summary, which I found to be clearer – “Calculates an equivalent interest rate when an investment increases in value”.
    (2) Returns. Delete “positive” as the function can return negative values depending on the relative sizes of PV and FV. Suggest changing “when an investment,PV, increases in value to FV in P number of periods” to “for the given arguments”. Maybe mention that by default the result is formatted as a percentage.
    (3) Arguments, P. Do you think it is worth adding that this argument may be entered as a fractional number, but in this case the result may be slightly inaccurate, as the fractional period will calculate as if partially compounded?
    (4) Arguments, PV. This seems to be too long and possibly incorrect (the only limitation is that PV must be non-zero; it can be either -ve or +ve). Suggest changing “present (current) value” to “present value of the investment (at the start of the term)”. Suggest deleting the 2nd and 3rd sentences. Maybe worth commenting that PV and FV should be in the same currency unit.
    (5) Arguments, FV. Suggest changing “determines what is desired as the cash value of the deposit” to “the target future value of the investment (at the end of the term)”. Maybe worth commenting that FV and PV should be in the same currency unit.
    (6) Arguments, 1st bullet. As mentioned in a previous comment, the only definitive condition for this argument is that P <> 0.0 (for obvious reasons if you consider the formula).
    (7) Arguments, 2nd bullet. Can I suggest that we delete this bullet and insert a suitable comment into the Additional details section (see below)?
    (8) Arguments, missing bullet. There should probably be another bullet which covers the disallowed case of P <= 0.0.
    (9) Additional details. Is the following paragraph worth inserting – “RRI calculates the fixed interest rate needed, so that PV invested for P periods (and compounded each period) is worth FV at the end of the term. RRI returns a fixed interest rate which applies to each period.”?
    (10) Additional details. The formula as written appears ambiguous to me – please insert round brackets around “FV/PV”.
    (11) Additional details. Suggest inserting a note such as – “The nature of the formula is such that a mathematical solution will not be possible for all combinations of the three parameters P, PV and FV. For cases having no solution, the function returns a numeric error (#NUM!)”.
    (12) Examples, 2nd example. This appears to be identical to the 1st example in all respects.
    (13) Examples, 3rd example. Suggest replacing “since there is a negative number raised to odd root. (-10)^(1/7)” with “, identical to the result of the formula =POWER(-10, 1/0.7)”.

    --Stevefanning (talk) 2020-11-26T11:40:09 (UTC)

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