Q: Does the coefficient of error has to be lower than 0.05?

A: There is not firm rule for what the coefficient of error “needs” to be. It can vary quite a bit depending on the CE method that you choose, and what the distribution of your cells looks like in your tissue. This is a very tricky question, and I would recommend checking out this help article: https://www.mbfbioscience.com/help/stereo_investigator/Default.htm#Stereology/Foundations/samplingPrecision.htm