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#1
05-16-2011
 USMLE Forums Guru Steps History: Not yet Posts: 324 Threads: 70 Thanked 239 Times in 144 Posts Reputation: 249
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
 The above post was thanked by: aknz (10-08-2011), ashishkabir (05-16-2011)

#2
05-16-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,268 Times in 881 Posts Reputation: 3278

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]
 The above post was thanked by: alicante03130 (01-26-2016), ashishkabir (05-16-2011)
#3
05-16-2011
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Quote:
 Originally Posted by bebix 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??/

#4
05-17-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,268 Times in 881 Posts Reputation: 3278

Quote:
 Originally Posted by drmdshah 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

 Tags Biostatistics-Epidemiology, Step-1-Questions

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