Definition
The observations are concentrated in data distributions near the mean, above the probability of being below the mean, also known as the "bell curve".
In detail
The normal distribution is a continuous probability distribution with a bell-shaped curve, where data is concentrated around the mean and symmetric. In quality management, it is used to describe natural variation and is the theoretical basis for control charts and Measurement System Analysis (MSA). In the system, it helps distinguish normal variation from abnormal variation, thereby determining whether improvement actions are needed. Unlike the uniform distribution (all values equally likely) and skewed distributions (asymmetric), the normal distribution assumes the superposition of many independent random factors. The output is typically the probability density function and control limit calculations.
Example in practice
A machining factory measured the diameters of a batch of shafts and found the data showed a bell-shaped distribution. They used normal distribution properties to calculate the mean and standard deviation, and set control limits. When subsequent sample points exceeded the control limits, they determined the process was abnormal and promptly investigated the cause. This helped them distinguish random errors from systematic deviations, effectively controlling product quality.