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Kevin Beckham Kevin Beckham is offline
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Default Margin of Error Formula

Is it valid to use
=CONFIDENCE(Alpha, Standard_dev, Size),
(returns the confidence interval for a population mean)
i.e.
=CONFIDENCE(0.05, 0.5, 16)
(= 24.5%) ?
Is the 1.96 factor also sample size dependent ?
(My stats are rusty)

Merry Christmas to all

Kevin Beckham

-----Original Message-----
the standard error for your sample percentage is =
sqrt(((100-percentage)*percentage)/n-1)

assume 50% is in A1, 16 in B1

= sqrt(((100%-A1)*A1)/(B1-1))

This comes out to 12.91%

assuming your percentage is normally distributed, then a

95% confidence
interval says you should go +/- 1.96 standard errors from

the mean

50% - (1.96 * 12.91%) as the lower bound and

50% + (1.96 * 12.91%) as the upper bound


(1.96 * 12.91%) = 25.303%

so you lower bound formula would be

= A1-1.96*sqrt(((100%-A1)*A1)/(B1-1))
your upper bound formula would be
= A1+1.96*sqrt(((100%-A1)*A1)/(B1-1))

--
Regards,
Tom Ogilvy


Heather Rabbitt wrote in message
. com...
Hi,

I'm looking for a formula in excel to give me the

maximum and minimum
margin of error at the 95% confidence interval for a

given percentage
and sample size.

For example the percentage may be 50% I have a sample

size of 16 and
using a stat testing program (STATCHCK) I know the

margin of error is
+/- 25% so my maximum would be 75% and my minimum would

be 25%.

My problem is I have over 10,000 numbers to check and I

want to
automate this in excel. I know there is a data analysis

add in excel
but not sure if it can be used to solve my problem.

Any help with my problem would be greatly appreciated.

If you think this should be posted somewhere else

please let me know.

Thanks in advance,

Heather



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