# Formula For Three Sigma Rule Of Standard Deviation

Sep 20, 2009I want to find the correct formula to find answers using excel for the folliowing:

1. >3Sds

2. 3SDs<>2SDs

3. 2SDs<>1SD

4. <1SDs

This question uses 3 sigma rule (see attached worksheet) and the formula needs to be re-arranged to meet new criteria. So, we need to consider both sides of nornmal distributiion (ND) curve. eg. when we say >3SD it means it should count all valuee falling in the rage "Mean(Average)+3SD AND Mean-3SD" i.e two extreme ends of the ND cureve. I have inserted a normal distribution diagram and colour coded the ranges so that it will be easy to identify. Please refer comments inserted in K7to N7, cells that contain formula that need to be reset.