# Confidence Interval Calculation

1. Jan 14, 2014

### MMCS

See attached for problem

Working:

Mean: 1203.26
Standard Deviation: 7.047

for 99% confidence interval, level of significance = 0.01
Therefore

0.5 - 0.01/2 = 0.45
Reading 0.45 from standard distribution table i get a value of 1.65

therefore confidence interval should be

1203.26 +/- (1.65*7.047)/√15

1203.26 +/- 3

however using this website to check my answer:

http://www.mccallum-layton.co.uk/tools/statistic-calculators/confidence-interval-for-mean-calculator/ [Broken]

It gives me a value of 5 (approx)

Thanks

#### Attached Files:

• ###### Untitled.png
File size:
9 KB
Views:
86
Last edited by a moderator: May 6, 2017
2. Jan 14, 2014

### Ray Vickson

For a (symmetric) 99% confidence on a quantity X, you want a 0.5% chance that X is below below the lower limit and a 0.5% chance that X is above the upper limit. In other words, if U is the upper limit you want Pr{ X ≤ U} = 1-0.005 = 0.995. What is the 99.5 percentile on the standard normal distribution?

Last edited by a moderator: May 6, 2017
3. Jan 14, 2014

### MMCS

Thanks for your reply, I'm not sure where the 0.005 value is from?

4. Jan 14, 2014

### Ray Vickson

What is meant by x%?