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#1
07-15-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,867 Threads: 149 Thanked 2,106 Times in 1,049 Posts Reputation: 2126
confidence interval for the population mean birth weight

In a health care utilization journal, results are reported from a study performed on a random sample of 100 deliveries at a large teaching hospital. The sample mean birth weight is reported as 120 ounces, and the sample standard deviation is 25 ounces. The researchers neglected to report a 95% confidence interval for the population birth weight (i.e.: mean birthweight for all deliveries in the hospital). You decide to do so, and find the 95% confidence interval for the population mean birth weight to be:

a) 119.5 ounces to 120.5 ounces
b) 115 ounces to 125 ounces
c) 70 ounces to 170 ounces
d) 117.5 ounces to 122.5 ounces

#2
07-15-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 646 Threads: 52 Thanked 762 Times in 256 Posts Reputation: 773

c) 70 ounces to 170 ounces
#3
07-15-2012
 USMLE Forums Addict Steps History: 1+CK+CS+3 Posts: 151 Threads: 48 Thanked 112 Times in 60 Posts Reputation: 122

mbbs2010...how did u get C...

I thot whenever one is given the sample size ...it must be used in calculating the CI

I think its B...115 to 125

#4
07-16-2012
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95% CI = mean +/- 2 x (SD/ sq root of n)

= 120 +/- 2 x ( 25 / sq root of 100 )

= 120 +/- 2 x 2.5 = 120 +/- 5 = 115 to 125 ounces

hope im right
 The above post was thanked by: tyagee (07-16-2012)
#5
07-16-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 646 Threads: 52 Thanked 762 Times in 256 Posts Reputation: 773

Quote:
 Originally Posted by stfidel mbbs2010...how did u get C... I thot whenever one is given the sample size ...it must be used in calculating the CI I think its B...115 to 125
Yup yup! U are right!
I dunno why I presumed it to be a normal distribution curve!
#6
07-16-2012
 USMLE Forums Master Steps History: --- Posts: 1,365 Threads: 648 Thanked 591 Times in 355 Posts Reputation: 601

Quote:
 Originally Posted by curiousmind 95% CI = mean +/- 2 x (SD/ sq root of n) = 120 +/- 2 x ( 25 / sq root of 100 ) = 120 +/- 2 x 2.5 = 120 +/- 5 = 115 to 125 ounces hope im right
Thanks. Hopefully this is correct.

 Tags Biostatistics-Epidemiology, Step-2-Questions

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