- CFA Exams
- CFA Level I Exam
- Topic 1. Quantitative Methods
- Learning Module 5. Sampling and Estimation
- Subject 5. Confidence Intervals for the Population Mean and Selection of Sample Size

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**CFA Practice Question**

For a normal population with s unknown, a sample of size n = 21 yields a sample mean of 32 and a sample standard deviation of 3. At a 90% confidence interval for m is ______ (to nearest 0.01).

B. 30.35 < m < 33.65

C. 30.87 < m < 33.13

A. 31.13 < m < 32.87

B. 30.35 < m < 33.65

C. 30.87 < m < 33.13

Correct Answer: C

Because s is unknown we use the t-distribution. We work with E as shown below. The a level is 0.1/2 = 0.05 and the degrees of freedom is 21 - 1 or 20. The table value is 1.725. E is 1.13 and the confidence interval is 30.87 < m < 33.13.

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**User Contributed Comments**
2

User |
Comment |
---|---|

PSVC |
How do we know when to use the one sided/two sided tails? |

ravinram |
Confidence interval is always 2 sided |