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Topic: Why multiply t by sample std dev?
Replies: 6   Last Post: Oct 2, 2012 6:42 PM

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Bruce Weaver

Posts: 752
Registered: 12/18/04
Re: Why multiply t by sample std dev?
Posted: Oct 2, 2012 9:38 AM
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On 01/10/2012 9:54 PM, Rich Ulrich wrote:
> On Mon, 1 Oct 2012 12:45:45 -0700 (PDT), Paul
> <paul.domaskis@gmail.com> wrote:

>> On Sun, 30 Sep 2012 12:55:11 -0700 (PDT), Paul
>> <paul.domas...@gmail.com> wrote:

> ... snip a bunch

>> Thanks, Dave, Rich,
>> I think I pin-pointed the source of my confusion. I didn't really
>> know what t was. It was just a curve and a table that we blindly used
>> according to a recipe. Well, Wikipedia

> (http://en.wikipedia.org/wiki/Student%27s_t-distribution) defined the
> t-value in a manner similar to

>> Rich:
>> [ (sample mean) - (population mean) ] / (sample standard deviation)

> No!
> It does not say that. It uses the sample standard ERROR.
> You are going to confuse yourself on into the future if you do
> not grasp the distinction and keep it clear.
> The Standard Error is the name used for the SD of a statistic.
> Yes, it is also a standard deviation. But the "SD of a sample"
> is distinct and larger than the "SD of a sample mean."

And to complete the thought, the (estimated) standard error (SE) of the
MEAN = the sample SD divided by the square root of the sample size.

Bruce Weaver
"When all else fails, RTFM."

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