Generating Double-Digit Random Numbers

I like =IF(RANDBETWEEN(1,2)=1,RANDBETWEEN(10,99),-RANDBETWEEN(10,99))

:-)

@Ken & @Rick

My guess is you two have been math majors of some type at some point in your lives...and most of what you know on this topic is well over the head of this simple MBA guy. For most of us, we look at the random numbers we're getting from Excel like inventory counters look at a pile of beans; they're "random enough". Not being flippant, just saying there are different perspectives.

Now, I'm smart enough to understand that if you multiple two random numbers the result is flawed because primes will be excluded except when one of them equals 1 (or negative 1) and the other is a prime.

That said, consider a binary random decision resulting in 1 or -1, and that result multiplying a ranged random number (RANDBETWEEN):

= IF(RANDBETWEEN(1,2)=1,-1,1)*RANDBETWEEN(10,99)

Why would that not produce a "uniformly random result"?

The product of two random variables with uniform distribution does not have uniform distribution. So while producing unpredictable values, none of these formulas that rely on two calls to RAND or RANDBETWEEN will produce a uniformly random result.

Allen's second formula:
=ROUND(RAND()*89+10,0)*((RAND()<0.5)*2-1)

does not produce a uniform distribution.
RAND()*89+10 produces a real number between 10 and 99 (call it x). ROUND(x,0) will produced the value 10 only if x is between 10 and 10.5. Similarly, ROUND(x,0) will produced the value 99 only if x is between 98.5 and 99. Thus 10 and 99 will be produced half as often as each of the other integers 11-98.

=TRUNC(RAND()*90+10,0)*((RAND()<0.5)*2-1)

should work

@BHershman,
"Excel, is capable to perform all kind of tasks - except three [two of which are illegal outside the State of Florida]".
It is very(!) easy to produce a range of Random numbers which will be UNIQUE (Never(!) repeat themselves) but this is, of course, another episode.
Michael (Micky) Avidan
“Microsoft® Answers" - Wiki author & Forums Moderator
“Microsoft®” MVP – Excel (2009-2015)
ISRAEL

Michael (Micky) Avidan:

I may have misread (again!) the details of the hint - which makes my comment irrelevant but NOT false. The essence of a random-number generator is that the chance of generating the SAME number twice in quick succession is very small.

I may be in need of a brain change but not a computer change.

@BHershman,
If the above proposed Formulas produces - every press on [F9] key (Reaalculation) - the same Random value then, to my opinion - with regret and sympathy - it is a bout time, for you, to replace your computer...Michael (Micky) Avidan
“Microsoft® Answers" - Wiki author & Forums Moderator
“Microsoft®” MVP – Excel (2009-2015)
ISRAEL

The essence of computer-generated RANDOM numbers is that each time the related function is called a DIFFERENT result is generated! Therefore any formula that makes 2 or more calls to a random function (as do all the formulas in the tip) cannot be expected to work as though each call produces the same test value!

The only reliable way of doing what is wanted is to write a user-defined function such as:
FUNCTION RAND2DIG() AS LONG
DIM RANDSTORE AS LONG
LOOP
RAND2DIG = WORKSHEETFUNCTION.RANDBETWEEN(-99,99)
UNTIL ABS(RAND2DIG) >= 10
END FUNCTION