Definition
Displays graphically the data distribution and the graph showing process variations.
In detail
A histogram is a chart that displays the distribution shape of data using bars, grouping continuous data into intervals and showing the frequency of data in each interval. In the quality management system, it is used to reveal process variation characteristics, such as distribution shape, central location, and dispersion, helping identify abnormal patterns like bimodality or skewness. Unlike a trend chart, a histogram does not focus on time order but on the overall distribution of data; compared to a control chart, it does not include upper and lower limits over time. The typical output is a distribution chart, accompanied by statistics such as mean and standard deviation, providing direction for subsequent cause analysis or process improvement.
Example in practice
An injection molding workshop collected part dimension data from the past week, totaling 50 measurements. The engineer grouped the data and drew a histogram, finding a clear bimodal distribution rather than the expected unimodal one. This suggested possible parameter inconsistency between two injection molding machines or operational differences between two shifts. The team conducted further stratified analysis, confirmed the root cause, adjusted equipment parameters, and restored the distribution to unimodal, improving process capability.