Q: In a group of 100 people, Avg GFR = 85ml/min with a SD of 2.5. Range for 90% confidence interval is:

a. 80-90

b. 81-89

c. 75-95

d. 70-100

  

 

 

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Ans: b.

 

 

By the property of Normal Distribution Curve:

Mean ± 1 SD covers 68% of the observations

Mean ± 2 SD covers 95% of the observations

Mean ± 3 SD covers 99% of the observations

Also,

Mean ± 1.65 SD covers 90% of the observations

 

Method 1: (Direct Method)

90% of observations will lie witin thge range of Mean ± 1.65 SD = 85 ± (1.65) (2.5) = 85 ± 4.125 = 80.875 to 89.125

Rounding this will give the answer as 81 to 89 (option b)

 

Method 2: (Indirect Method)

68% of people will lie within Mean +/- 1SD = 85 +/- 2.5 = 82.5 to 87.5

95% people will lie within Mean +/- 2SD = 85 +/- 5 = 80 to 90

 

Now 90% of people will lie between the ranges for 68% and 95%.

There is only one option given which has lower limits between 80 and 82.5… and upper limit between 87.5 and 90… So Answer is b