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11#
发表于 2011-7-13 16:20
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I will answer your 2nd question first and use that explanation to answer your 1st question.
2) Isn't SD of sample = SE?
The answer is NO. I guess an example would be better to explain this.
There are totally 1000 L1 candidates registered for Dec 2009 exam. Their Mean age is 28. And age of the youngest candidate is say 20 and that of eldest candidate is say 50. So, this is your POPULATION with Mean 28 and SD of 30 (i.e. 50 - 20). (though it is not a simple subtraction, but doing it just to give an idea of width)
Next, say you dont have the time to get ages of all candidates and instead, you decide to take a sample of 100 candidates in NYC area. Their mean age comes out to be 25. And youngest and eldest in this Sample is 21 and 49. So, this is one SAMPLE with Mean of 25 and SD of 28 (i.e. 49 - 21)
Now, lets say, there are 20 such samples (though you dont need more than 1 sample). If you plot a distribution of MEAN Ages from all these 20 Samples, it will have some Mean and some Deviation of its own. Deviation of such a distribution is known as STANDARD ERROR.
And by formula, SE in this case would be 30 / sqrt of 100 = 3
And if Pop SD was not known, then it would be 28 / Sqrt of 100 = 2.8
So, the answer is, SD of Population or a Sample is the spread of values in that Population or Sample. Whereas, SE is the spread of MEANs coming from various Samples from a given Population.
Always remember that, Population or Sample Distribution will be lot more thicker than the Distribution of MEANSs coming from various samples. Hence, SD of Population or Sample will be MUCH MORE than the SE.
Now your 1st Question:
1) When do we divide by SE and when by SD?
When you have your Sample Mean and you want to know the extent to which this Sample Mean represents (estimates) the Population Mean, you will use SE.
In all other cases, you will either use SD of Population or SD of Sample, whichever is available.
Hope this clarifies. I will be glad to explain further if it is not so clear.
Edited 1 time(s). Last edit at Monday, October 12, 2009 at 03:54AM by rus1bus. |
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