Often a high confidence level (95 or 99%), means that your interval must be very large (high margin of error). Ultimately, we would like to create a confidence interval with a high confidence level and very small margin of error.

How can we control that???Make the z* value

smaller that causes you to accept a lower

confidence level.

Make the s value smaller

this does make it

easier to get a more

accurate m, but is difficult

to do

Make the n (sample size)

larger

dividing by a larger number makes

smaller and in turn the margin of error smaller

n

Best Option!

So how do we find the desired sample size?Let m = the desired margin of error

So,

Now solve for n.m

nz

Pg 640 #20b 

The sample size must be at least 2653 people

Divide by 2.575

Multiply by and then divide by 1.9417

Square both sides to clear the radical