Statistical methods for research workers

Excerpts

The value for which P = 0.05, or 1 in 20, is 1'96 or nearly 2; it is convenient to take this point as a limit in judging whether a deviation is to be considered significant or not. Deviations exceeding twice the standard deviation are thus formally regarded as significant.

Reference

Ronald A Fisher “Statistical methods for research workers” (1950) DOI: 10.1007/978-1-4612-4380-9_6

@Book{fisher1950,
  title = {Statistical methods for research workers},
  author = {Fisher, Ronald A},
  year = {1950},
  publisher = {Springer},
  doi = {10.1007/978-1-4612-4380-9_6}
}