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Confidence intervals - Normal distribution

Hi everyone,

I'm really struggling with confidence intervals using normal distribution. Please can someone explain this to me.

Thank you :smile:
Do you have an example question?
Original post by science_geeks
Hi everyone,

I'm really struggling with confidence intervals using normal distribution. Please can someone explain this to me.

Thank you :smile:
Original post by kungfuslothy
Do you have an example question?


Yeah, Its this question hereTSR .jpg
I'm a first-year Economics student and I'm assuming two-sided confidence interval means +/- answers
For a 95% confidence interval, the equation would be X+/-(zo/root n)
X - mean
z- critical value at 5% sig (which is 1.96 in this case)
o - standard deviation
n - sample size
So it would be 1465+/-((1.96*108)/root 20) = (1417.67, 1512.33)
I usually put the negative answer first in the brackets cause that's what the mark scheme says for me but idk for you.
Original post by science_geeks
Yeah, Its this question hereTSR .jpg
Ah yeah, I think you're right because the other questions I've seen only ask for 2 sided confidence intervals too. How do you find out what the critical value is at z? Is it by the % points table?

Also, just want to make sure but the relationship between standard deviation and variance is that the SD^2 = Var. Thank you so much though! That makes so much more sense :smile:

Original post by kungfuslothy
I'm a first-year Economics student and I'm assuming two-sided confidence interval means +/- answers
For a 95% confidence interval, the equation would be X+/-(zo/root n)
X - mean
z- critical value at 5% sig (which is 1.96 in this case)
o - standard deviation
n - sample size
So it would be 1465+/-((1.96*108)/root 20) = (1417.67, 1512.33)
I usually put the negative answer first in the brackets cause that's what the mark scheme says for me but idk for you.
The critical value is in the two tail probability table under the % points table
if it was 95% confidence you'd look under 0.05 etc
And yes root Variance = SD
Original post by science_geeks
Ah yeah, I think you're right because the other questions I've seen only ask for 2 sided confidence intervals too. How do you find out what the critical value is at z? Is it by the % points table?

Also, just want to make sure but the relationship between standard deviation and variance is that the SD^2 = Var. Thank you so much though! That makes so much more sense :smile:

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