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Old 05-16-2011
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Stats Calculating the Standard Error of Mean and Confidence Intervals?

A research study is designed to identify the mean body weight of women btwn the ages of 30 and 39.To obtai this,a researcher obtains the body weights of unbiased sample of 81 women in this age group.The mean body wt is 135pounds with a standard deviation of 18.

What is the estimated standard error of the mean for this population?
A. 0.05
B. 0.10
C. 1.0
D. 2.0

2. What are, respectively, the 95% and 99% confidence intervals for this sample?
A. 131-139 AND 130-140
B. 130-140 AND 131-139
C. 129-141 AND 130-135
D. 130-135 AND 129-141
E. 131-139 AND 129-141
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Old 05-16-2011
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SE = sd/sqrt(n)
D) 2.0

95% = Z alpha/2 : 1.96 (close to 2)
99% = Z alpha/2 : 2.58 (close to 2.6)

CI = mean +or- Z x SE
mean = 135
SE = 2

95%CI : 131 - 139 [131.08, 138.92]
99%CI : 130 - 140 [129.8, 140.2]
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Old 05-16-2011
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Quote:
Originally Posted by bebix View Post
SE = sd/sqrt(n)
D) 2.0

95% = Z alpha/2 : 1.96 (close to 2)
99% = Z alpha/2 : 2.58 (close to 2.6)

CI = mean +or- Z x SE
mean = 135
SE = 2

95%CI : 131 - 139 [131.08, 138.92]
99%CI : 130 - 140 [129.8, 140.2]
hey..what is sqrt(n)?how did u get answer 2??pls explain in detail pls......and also learn me how did u get Z value??/
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Old 05-17-2011
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Quote:
Originally Posted by drmdshah View Post
hey..what is sqrt(n)?how did u get answer 2??pls explain in detail pls......and also learn me how did u get Z value??/
Question 2:

sqrt = square root

2 formulas you must know:
1.- SE = SD / Square root (n)
2.- CI = mean + or - [Z score * SE]

There is not a formula for Z scores - you have to memorize the values.

for the 95% CI:

mean = 135
SE = 2
Z score (5% two tailed) = 1.96 = 2.0

135 - [2*2] = 131
135 + [2*2] = 139
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