Nicely said; but, Hey! off topic... and yet you went on with; "Why does UNIDO use it - maybe because it's built in to Excel? ;o) Maybe because they are bureaucrats that need to get paid for the quantity of cases they evaluate? Maybe because the guys deciding about the rules of evaluation got their education quite some time ago and science went on - frankly, I don't know.
What I do know from the projects I was part in (among them an investment project with almost 2 billion EUR in volume - initially, before we helped our client to save 30% of it), that it's not unusual that you need to explain a corporate decision maker why something like discounting is a good idea at all..."

Matter of fact, it was not only the UNIDO; but, also and more so the Inter-American Development Bank - IDB (
http://www.iadb.org ) and it will appear quite incomprehensible the down and out people in the Latin American and Caribbean countries that the Inter-American Development Bank serves quite fit the bureaucratic labeling.

.
Re: "maybe because it's built in to Excel?". The Internal Rate of Return (IRR) functionality as far as I do know comes integrated into
the IBM's Lotus 1-2-3,
the IBM's Lotus Notes Productivity Suite, IBM's Lotus Symphony and Open Office's - Calc Spread Sheet Applications; so, it is not the case that the Internal Rate of Return (IRR) is restricted to the Microsoft's Excel Application alone.
Then again; are we done now with the question or what - stopping at the Internal Rate of Return (IRR) computations... what about the question with regards to the ability to convert from the Internal Rate of Return (IRR) values to the Economic Rate of Return (ERR) values during the evaluation of projects requiring international considerations.
Just curious.. (re: "What I do know from the projects I was part in (among them an investment project with almost 2 billion EUR in volume") which country are you in and what is it that you do at present (job wise

.
Also, do you have any idea as to why the feasibility of some particular projects have to take as much as three years to be completed.

Maybe your expertise is needed here.

!
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