Without knowledge of e r and e s , T can never be accurately measured. This potentially is a huge problem. What is e r Random errors increase the dispersion. These errors are associated with apparatus or method used in obtaining the data.
All data sampling is subject to random error, period.
There is no way to avoid it. Here is how you due to subtraction. In a bell-shaped distribution, the various components are added or subtracted in quadrature. Systematic errors mean that different methods of measurement are being applied to the same variable. This means that the position of the mean is strongly effect.