Visual | Bell Curve

Bell curve comes from the Normal or Gaussian distribution in Statistics. A bell curve is symmetrical on both sides of the mean value μ. The curve’s standard deviation is σ. The area under the curve represents the probability. 

Scientists have found that many continuous data and psychological symptoms follow the normal distribution, such as height, weight, blood pressure, and even IQ.
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The bell curve is also the basis for the Six Sigma theory, where Six Sigma means the probability of having products free of defects is within six times of the bell curve’s standard deviation σ. Based on the normal distribution theory, the probability within 6σ is 99.99966%, which means there are only 3.4 defects per million opportunities (DPMO) of making defects. Six Sigma theory provides a set of techniques and tools to ensure the process can reach the goal of 99.99966% products without defects.