1. The problem statement, all variables and given/known data I have 20 data points of the surface roughness of oil pipes, the mean, and the std of the sample. I need to get a 95% confidence interval of the mean surface roughness of these pipes. I did a boxplot, and found that it's negatively skewed, and therefore not normally distributed: Would using the t-table be OK? I'm usually used to doing this for data when it is normally distributed, I'm not too sure what to do when it's not normal. This is what I did: x ± t-val(s/√n) Where the t-val is t19 and I looked up the t-table for the value. s = std and n = 20 and x is of course the mean.