Control Charts

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Control Charts

Control charts, also known as Shewhart charts are tools used to determine if a manufacturing or business process is in a state of statistical control. The control chart was invented by Walter A. Shewhart, (also known as the father of statistical quality control) while working for Bell Labs in the 1920s. The company's engineers were seeking to improve the reliability of their telephony transmission systems. The engineers had realized the importance of reducing variation in a manufacturing process. Shewhart framed the problem in terms of Common and special causes of variation and in 1924 introduced the control chart as a tool for distinguishing between the two. Dr. Shewhart created the basis for the control chart and the concept of a state of statistical control by carefully designed experiments. Control charts in simpler terms are graphs used to study how a process changes over time. Data are plotted in time order. A control chart consists of a central line for the average, an upper line for the upper control limit and a lower line for the lower control limit. These lines are determined from historical data. It includes points representing a statistic of measurements including things like mean, range or proportion of a quality characteristic in samples taken from the process at different times the data. By comparing current data to these historical lines, one can draw conclusions about whether the process variation is consistent (in control) or is unpredictable (out of control). The standard or standard deviation of the statistic is also calculated using all the samples. The chart may have other optional features, including labels such as upper and lower warning limits, drawn as separate lines, typically two standard errors above and below the centre line. When interpreting control charts, we must understand that it represents a picture of a process overtime. To effectively use control charts, one must be able to interpret the picture.

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