two standard deviations

https://en.wikipedia.org/wiki/Standard_deviation

• In science, many researchers report the standard deviation of experimental data, and only effects that fall much farther than two standard deviations away from what would have been expected are considered statistically significant—normal random error or variation in the measurements is in this way distinguished from likely genuine effects or associations.
• One SD -> 68%
• Two SD → 95%
• Three SD → 99.7%

Dark blue is one standard deviation on either side of the mean. For the normal distribution, this accounts for 68.27 percent of the set; while two standard deviations from the mean (medium and dark blue) account for 95.45 percent; three standard deviations (light, medium, and dark blue) account for 99.73 percent; and four standard deviations account for 99.994 percent. The two points of the curve that are one standard deviation from the mean are also the inflection points.                                                                                                                                                                                        

about 95 percent are within two standard deviations (μ ± 2σ)