(bong — email arrives)
Hi Gordon! I have a question about the difference between variance and
standard deviation…seems they are basically the same thing? also,
how is it that a higher SD = higher risk when 6SD or Six Sigma is a
Thanks for any clarification you can provide!
Great questions. Heady questions. Whew!
I am not a stat guy, but try this on for size:
Standard deviation, as you mentioned, shows how much range exists in a
group of measurements / estimates ( good stuff to know when you are
evaluating someone’s claims about money or results).
Variance takes that “raw number” and converts (squaring it, again, you
were right on point) it into a unit that can used to crunch data …
for instance summing all variances in a sample and taking the square
root of that sum, for the PMP exam, gives you task variance, which is
helpful to guess if a group of estimates seems achievable at various
levels of confidence.
And then it shifts gears, logically.
Six Sigma looks at process variation (bad! bad!) and seeks to decrease
the standard deviation of a process (fewer items that stray from the
desired outcome, the process mean). Visualize it like this …
Extend your right thumb and finger so they look like a slope (turn
your hand sideways). Now, move your finger and thumb closer together.
Viola! You have decreased the spread, decreased standard deviation!
Ok, so how does that relate to Six Sigma? If the customer specs are
moved tighter (pinch the fingers) and you operate your process under
tighter controls (hopefully, meeting the customer expectations /
specs). With Six Sigma you create a very consistent process close to
your target. Very little rework is necessary. Happiness abounds.
In theory, that is …
Good news is that, if you can warp your head around this, you will have several questions on the exam that relate to these concepts across the Quality, Time and Cost knowledge areas.
Thanks for the questions!
You are great!
P.S. if you are in the mood for a gratuitous mnemonic: Remember the skit from Kids in the Hall, “pinch your head, pinch your head?” If you change the words to “pinch your spread, pinch your spread” and imagine yourself looking at a bell-shaped curve of data, pushing the data closer to the mean, you increase your chances of shoving this conversation into long-term memory. Enjoy!
P.S.S Those of you who follow Kids in Hall, I look forward to the chatter about my artistic inaccuracy.