On our study guide, my teacher asks the question: “What is z for a two tail 95% confidence interval.” Does this mean I take the percision value of 1.96, or do I take 100% - 95% and divide that answer by two to see how much is left in each tail? Or am I just completely off base?
Thanks.
You may have a table on which you can look this up directly, in which case, just take what the table gives you.
If you have to figure it out for yourself, though, yeah, start with 100%-95% = 5% in the two tails, so 2.5% is in each tail, then figure out what z would be so that 2.5% of the area under the standard normal curve is out in the tail past that z.
Good info, it’s also ironic that I’m actually from around Springfield. Using an “Areas Under the Normal Curve” chart, I can use the value of 1.96 for my z and get an answer of .4750. I take it that this answer is the total amount in each tail, so I would divide by 2 in order to get the proper amount in each tail. Am I right? If so would the z value for 95% CI on a one tailed test would just be .4750/1? Thanks.
OK - now I’m really confused. I remember that for my first stats class there was a table that read:
90% CI = 1.64 z value
95% CI = 1.96 z value
99% CI = 2.54 z value
With this being said, for the question “what is the z value for a 2 tail 95% CI?” wouldn’t the answer being 1.96. Does it even matter if it is one tailed or two tailed.
Wouldn’t you take your z value and find it in the chart labled “Areas Under The Normal Curve” in order to find out what percent of data fell into your confidence interval? Thanks
