terça-feira, setembro 05, 2006

PMP Question - Standard Error

Question :

"A process characteristic has a true mean of 150 and a true standard deviation of 20. A sample of 25 items is taken from this process. The standard error of the estimate is:
A. 0.2
B. 0.8
C. 4
D. 5
E. 7.5"

Correct Answer is C.

The Standard Error of a statistic is the “Standard Deviation” of the “sampling distribution” of that statistic. Standard errors are important because they reflect how much sampling fluctuation a statistic will show. The standard error of a statistic depends on the sample size. In general, the larger the sample size, the smaller the standard error. The standard error of a statistic is usually designated by the Greek letter sigma (sigma) with a subscript indicating the statistic. The formula for the standard error of the mean is:

where sigma ( numerator ) is the standard deviation of the original distribution and N is the sample size (the number of scores each mean is based upon).

So, Standard Error here is 20/sqrt(25) = 20/5 = 4. That is C !

The catch here was to know that a "standard error" is the same as "standard deviation". Tricky ...

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