You can look up CJJ-90 on the Internet and calculate it according to the standard deviation calculation formula in the back pages of this book. Mean error, variance and standard deviation are some concepts of probability, and it is too troublesome to deduce the formula. It is better to look up the corresponding book instructions directly on the Internet. There is a saying: engineers are not smart, as long as they can turn the table. You just need to remember the origin of these formulas. There are no ready-made specifications on the Internet, and now they can basically be found in Baidu Library.