In: Statistics and Probability
the age when smokers first start from previous studies is normally distributed with a population standard deviation of 1.8years old. A survey of smokers of this generation was done to estimate the mean age. the sample of 35 smokers found that their mean starting age was 14.7year old. find the 95% confidence interval of the mean
Solution:
Note that, Population standard deviation()
is known..So we use z distribution.
Our aim is to construct 95% confidence interval.
c = 0.95
= 1- c = 1- 0.95 = 0.05
/2
= 0.05
2 = 0.025 and 1-
/2 = 0.975
Search the probability 0.975 in the Z table and see corresponding z value
= 1.96
The margin of error is given by
E = /2
* (
/
n )
= 1.96 * (1.8 /
35)
= 0.59634084213
Now , confidence interval for mean()
is given by:
(
- E ) <
< (
+ E)
(14.7 -
0.59634084213) <
< (14.7 + 0.59634084213)
14.1037 <
< 15.2963
Required 95% Confidence interval is (14.1037 , 15.2963)