I have experimenatal data from medical industry. In the experiment 4 observations were weighted, each one 2 times for more precise measurement.

As a final weight, a mean from each pair is reported. So I have 8 values, a pair (2) for each observation and 4 means.

We assume a normal distrubution with unknown mean and standard deviation.

I need to calculate 95% CI for mean weight. I use this formula: $ $ X¯±t∗S/\sqrt{n}, $ $ where where S is the sample standard deviation and where $ t$ cuts probability $ 0.025$ from the upper tail of Student’s t distribution with $ n−1$

degrees of freedom.

Should I do this from 8 values (7 df, n=8) or from calculated mean values from each pair(3 df, n=4)?

Maybe a should use another formula? Why?