# Help

#### 1NAMILL

##### New Member
This is the 2nd question:

Construct a confidence interval for
the mean of the differences d for the population of paired data. Assume that the population of paired differences is normally distributed.

mean of difference = 3.125
sd of the dufference = 2.911
n = 8
determine a 90 percent confidence interval for the mean of the difference. Fill in the lower value:

________ < mean of difference < 5.075

use 3 decimal places.

This is what I did:

E = t a/2 sd/sqrt of n =

The degrees of freedom is 8-1 = 7. However, as I look at the table for a 90% confidence interval under the critical t values, I'm stuck. I know that it's 0.01. However, I don't know if I should look under the area in one-tail or the area in two-tail. This is where I get lost. I don't know where to look in the table. If I can find the critical t value for a 90% confidence interval - I'll be ok. Please advise.

#### JohnM

##### TS Contributor
Look up t under a 2-tail confidence interval.

For a 2-tail 90% confidence interval, it's not .01, but .1 divided by 2, which gives you .05.