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standard error of the mean help - i don't really know what it is. watch

1. hi, i have my biology ISA tomorrow and i know that the statistical test is doing the standard error of the mean.

i know that i have to do:

SD/sqrtN

but what does it actually show? i know that you use it to compare 2 means (thats a question in my ISA paper my teacher told me). but why? i can do the calculation easily but i don't know what the answer that i get actually is telling me. could someone please explain this?

also, we have to do 95% confidence intervals.

what does this mean?

we always have to find something to do with 5% intervals. i think that it means that the probability of the data being down to chance is less than 5%. but why choose 5% over the other values you are given like 6,7,8,9%?

any help would really be appreciated.
2. If (a,b) is 95% confidence interval for the mean, it's saying that we are 95% certain that this interval contains the population mean, Mu.

SD/sqrtN measures the error in using the sample mean as an estimate for the population mean, Mu.
The bigger the sample size N, the smaller SD/sqrtN is, so this makes sense intuitively.

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Updated: March 29, 2011
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