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Statistics: Power and sample size question watch

1. Minitab tells me the power of a test with n=9 and a difference of 8 with standard deviation 9.88 is 0.680537

When I work it out by hand, I get 0.0075. I am using the formula:

zß = ( x(bar) - mu ) / (variance/sqrt(n))
With a hypothesised mean of 172 and an actual mean of 180, this gives

= (172-180)/3.29333 = 2.429 which gives ß = 0.9925 which gives a power of 0.0075.

This is the equation I have been taught to use, so i'm confused as to why my result differs by such a huge amount to the minitab output.

Any help would be appreciated.
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7. (Original post by Lewk)
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I don't know anything about Minitab.
It may help to see the input and output and I'll see what, if anything, I can do.

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Updated: May 22, 2013
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