# Help with Testing for Differences Between Proportions with Independent Samples

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• #235828

tgause
Participant

I’m teaching myself about testing for differences between proportions with independent samples. I use Minitab 17. I found an example in Minitab that I thought I would try to calculate by hand to see if I could get the same result to confirm my understanding of the formulas. As you can see in the example, the z-score Minitab got was 0.58. Yet, when I plugged in the numbers to the formulas they used, both times I got a z-score of 0.61. What am I doing wrong? I calculated this example both ways: first, using the pooled estimate of p for the test, and second, using the separate estimates of p for each population. Both times, I got a z-score of 0.61.

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#235833

Robert Butler
Participant

.002112 + .002688 = .0048800 and the square root of that is .069282. Divide .04 by that and you get .577350 which when rounded up to the level of precision allowed in your calculation is .58

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#235834

Robert Butler
Participant

Hmmmmmm…there must be some kind of double precision inside the calculator. If I first sum the two values and take the square root of the sum and do a division of that number into .04 I get the numbers indicated. However, if I just input .00488 and take the square root of that number I get .069857 which when divided into .04 gives .572 which doesn’t round up to .58.

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#235835

Robert Butler
Participant

Never mind – it always helps to correctly read your own calculator screen – the sum is .004800 not .004880. When you take the square root of .004800 and divide .04 by that you get .577 which rounds up to .58

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#235837

Mike Carnell
Participant

@rbutler You never cease to amaze me.

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#235895

tgause
Participant

@rbutler, I found the problem. After 10 minutes of pulling my hair out and doing the calculation over and over and over and always getting the same result of 0.61 (definition of insanity, right?), I decided to put on my Green Belt hat and do a root cause analysis. What could be the root cause of this problem? So, decided to do an experiment of sorts. I did the calculation on a different calculator, thinking, maybe…just maybe…something weird is going on. And sure enough, something weird was going on. It wasn’t my calculations. It was my calculator. When I typed in all 9’s, I discovered the LCDscreen is messed up. The 4th digit from the left appears as a 5, not a 9! Calculator’s in the trash. Thanks!

• This reply was modified 1 year, 7 months ago by tgause.
• This reply was modified 1 year, 7 months ago by tgause.
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