Tuesday, August 13, 2013

Estimation math made easy via zequals

How precise are your estimates?  Are we inferring false precision with those 5 significant digits?

I saw an hours estimate for a medium size project at our company just the other day.  I don't remember the actual digits, but that's not the point it was a number like:
4145.8 hours
Let's just assume that a team of 7 people will do this project.  How many weeks is this?  I'd guess 4145.8 hr divided by (7 * 40) equals 14.806 weeks.  Or, as a human would say it 15 weeks, give or take.

Are those digits really necessary?  No.  Are they even real?  No.  They are just artifacts of inappropriate rounding or truncation.   There is very little doubt that someone could estimate a 4000 hour project down to the quarter of an hour (the implied accuracy of the 0.8 hour).  And if you think they are so good at estimating that they can distinguish between 45 hours and 50 hours for a portion of this project - well, then keep on trucking to the minute, or second.  There is a problem with your math.

Your solution is EASY.


So here's a solution that makes the calculator obsolete - estimation is an art - learn to use it in your math.  Learn the power of ruthless rounding via the technique called zequals by Rob Eastaway.  Zequals is a math symbol much like the equals sign.  And means much the same thing.  What is on one side of an equation is zequals (or almost equal) to the other side of the equation.  The symbol is: (insert graphic here), but since I don't have one on my keyboard I'm going to use z= to represent zequals.  It is a method of approximating each term in the equation by it's rounded value to one significant digit and then performing the math.  An example right about now would be helpful.

7 * 8 = 56  but 7 * 8 z= 60.   Perhaps another example will help.

13 * 38 = 494 but 13 * 38 z= 10 * 40 z= 400.   Are you thinking this is not very accurate or precise?  Then look here on Periodic Videos for an error graphic. Now, let's try some bigger numbers.

456 * 378 = 172368  however 456 z= 500 and 378 z= 400 and 5*4 = 20 so add in the other zeros, that 4 more zeros and you get 20,0000.  Now do you see how easy zequal makes math of estimates?

Here watch Rob explain it, now that your interested.




Will you start doing your project estimation math using zequals?  I will.  Thanks Rob.  Here's his math book for moms and dads.

See Also:

Relearning to Count - Zero, One, Many


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