# Statistics: Confidence Intervals

1. Jul 7, 2008

### Phrynichos

The problem in question is as follows:

Find themargin of error for the given values of c, s, and n. where c is the confidence interval, s is the sample standard deviation and n is the number of objects.

c=0.65 s= 1.5 n=50

the formula: E= (zc s) / n^.5

Level of Confidence Chart

c ........................zc
.80.......................1.28
.90.......................1.645
.95........................1.96
.99.........................2.575

As you can see, the zc value for c=.65 is not present on the chart. I don't know how to proceed to solve the problem by finding the corresponding zc for when c=.65. if anyone could show me, that would be appreciated.
thankx

2. Aug 13, 2008

### learnfrench

$$z_{c}=0.93$$ for $$c=0.65$$. Easy to find on your table for the Standard Normal Distribution.

3. Aug 30, 2008

### HF08

Hey LearnFrench,

How do they determine the 0.93 statistic in the first place? Assuming I have no life, how could I derive a table on my own?

Thanks,
HF08

4. Aug 31, 2008

### CRGreathouse

Numerical integration of the normal distribution's pdf would probably be the easiest way. The more traditional but also more complex way would start with the error function.