Right, I understand that a confidence interval is calculated from:
Mean +/- Critical value * Standard error
What I don't understand is how you calculate the critical value for a given confidence interval. I realise that for a 95% confidence interval the critical value is +/- 1.96 but I don't understand how this is calculated or how to calculate it for other percentages. Can anyone help me?
(I've tried looking through the formula booklet but I can't see the critical values tabulated anywhere)