06. SPC – Histogram

Histogram is part of the data display tools for SPC, and generally used for the following purpose:

    • Observe variation and frequency of occurrence within the data.
    • Interpret data distribution and predict potential trend of process performance.
    • Help to indicate if there are any changes within the process. 
    • Determine if process is capable to meet customer requirement.

Histogram can be interpreted based on process data distribution (data points per group) based on the specification median, upper spec limit (USL) and lower spec limit (LSL). The following charts are demonstration with respect to the process distribution along with countermeasures for each situation.

The following below are displayed SPC scenarios and respective countermeasure methods to bring process back to idea state.

The following shapes are general SPC phenomenon between specification limits.

There are 5 types of common distribution observed for histogram as following, and some process is naturally skewed so normal distribution might not be the default distribution for the process results.

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