quick stats question

[Guest]

What is the value of za/2 for an 80% confidence interval?
a. .255.
b. 1.28.
c. .0398.
d. .525.

i know the answer is a, but does anyone know how to calculate that ?

 

[tkhunny]

Calculate what? Look it up in a table or get your calculator to tell you.

80% suggests (100% - 80%)/2 = 10% -- Find the z-value that leaves 10% in the tail region.

 

[Guest]

ahh i see, thanks tkhunny

i didn't know calculators can tell you this though, do you know any models in particular that have this function?

 

[tkhunny]

Many calculators have Probability Density Functions and Cummulative Probability Density. Some even have the inverse function available. You should be able to find references in the Owners Manual.