Confusion about how to look up t value for margin of error in confidence intervals I have been given the formula for a confidence interval on a gaussian distribution with unknown variance as 
$\bar{x} \pm t_{\frac{\alpha}{2},n-1} \frac{S}{\sqrt{n}}$
And I am wondering which column to use in the T table for a 95% CI.

I suspect it must be $t_{.995}$ here
But am wondering how to reconcile the notation with the $t_{.025}$ implied by the formula I have been given. ( I am thinking $\alpha$ would be .05 )

There are several ways of describing this:

*

*A $95\%$  confidence corresponds to a probability of $1-\alpha = 0.95$ of the specified interval covering the parameter


*which corresponds to a probability of $\alpha=0.05$ of a $t$-distributed random variable being outside the interval,


*and here you want two tails, so by symmetry a probability of $\frac \alpha 2 = 0.025$ of being in the upper tail


*with a probability of $1-\frac \alpha 2=0.975$ of being below the upper point of the desired interval
You want the corresponding column labelled cum.prob $t_{.975}$ or one-tail $0.025$ or two-tails $0.05$
