Used in basic data analysis.
A central tendency is rarely perfectly centered. Even if it was, it wouldn’t stay that way. As time goes on a standard deviation can drift over 1.5 sigma over time.
If there was no 1.5 sigma drive a 6 Sigma process would only generate 2 defects per BILLION! As it is, because of this drift, the errors are 3.4 per million.
http://www.six-sigma-material.com/Central-Tendency.html (Measures of central tendency)