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Hoping someone can clear up what I hope is an easy solution. I'm trying to do a confidence interval (95%) with 67 degrees of freedom. For my T (Critical) I got 1.668 but when I do my calculations, I get -4130109 +- 2,832,883, however, the answer is wrong. I'm thinking that the issue is with my critical t. Since my DOF is large at 67, should I be using a z instead of t? If so, does anyone know the proper formula?enter image description here

Thanks!

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You are using the wrong value for the critical value of t.

You want the value of t where 2.5% of the distribution is larger than t, 2.5% is smaller than -t, so 95% of the distribution is between -t and t.

What you have (1.668) is the value of t where 5.0% of the distribution is larger.

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