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#1
07-15-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,867 Threads: 149 Thanked 2,088 Times in 1,047 Posts Reputation: 2108
95% Of the Population

A random sample of 100 12-year-old boys were chosen and the heights of these 100 boys recorded. The sample mean height is 64 inches, and the sample standard deviation is 5 inches. Assuming heights of 12-year-old boys are normally distributed. Which interval below includes approximately 95% of the heights of 12-year-old boys?
a. 63 to 65 inches.
b. 39 to 89 inches.
c. 54 to 74 inches.
d. 59 to 69 inches.
e. Cannot be determined from the information given.
f. Can be determined from the information given, but none of the above choices is correct.

#2
07-15-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 646 Threads: 52 Thanked 761 Times in 256 Posts Reputation: 772

because this is a normal distribution curve ...
mean=median=mode

in such a curve 95% CI is the mean +/- 2 SD

ie 64 +/- 2x5

ie 64+/- 10

so 54--74 ie C
#3
07-16-2012
 USMLE Forums Addict Steps History: 1+CK+CS+3 Posts: 151 Threads: 48 Thanked 109 Times in 60 Posts Reputation: 119

why not A.....using standard error of mean?
#4
07-16-2012
 USMLE Forums Master Steps History: --- Posts: 1,365 Threads: 648 Thanked 585 Times in 354 Posts Reputation: 595

Quote:
 Originally Posted by stfidel why not A.....using standard error of mean?
What's that
#5
07-16-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 189 Threads: 20 Thanked 506 Times in 99 Posts Reputation: 516

For interval we use the formula
interval = mean +/- Z(sd/n½)
so interval = 64 +/- 2(5/10) {at 95 % ci z = 2)
= 63 or 65
so A
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#6
07-16-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 646 Threads: 52 Thanked 761 Times in 256 Posts Reputation: 772

Quote:
 Originally Posted by mayursn39 For interval we use the formula interval = mean +/- Z(sd/n½) so interval = 64 +/- 2(5/10) {at 95 % ci z = 2) = 63 or 65 so A
but the question says that it is a normal distribution curve.
so u just use the +/- 2 SD formula for 95% CI
right??
#7
07-16-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 189 Threads: 20 Thanked 506 Times in 99 Posts Reputation: 516

No idea.. I guess you are right .. May be this formula doesnt apply for Normal distribution.. will have to check it.
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Don`t Let Others Tell You What You Cannot Do !!!!
#8
07-16-2012
 USMLE Forums Addict Steps History: 1+CK+CS+3 Posts: 151 Threads: 48 Thanked 109 Times in 60 Posts Reputation: 119

Quote:
 Originally Posted by tyagee What's that
at tyagee...i meant the formula used below by others to arrive at the option A...

I have always known wen u are given the sample size population u have to use the said formula...

If not...use Mbbs2010 direct method ...cheers
 The above post was thanked by: mbbs2010 (07-16-2012)

 Tags Biostatistics-Epidemiology, Step-2-Questions

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