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Identifying Patterns on Control Charts

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  • 0:01 Control Charts
  • 1:00 Cycles
  • 2:06 Trends
  • 3:33 Lesson Summary
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Lesson Transcript
Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

Control charts can be valuable for businesses to assess how they are doing, but how do you interpret them? In this lesson, we'll examine common patterns that can be identified when examining control charts.

Control Charts

Ling owns a company that makes scented candles in cute jars and tins. Mostly, her candles are very good, but every so often a candle comes off the assembly line with a broken jar or a wick that's too short. Ling wonders if she should be worried about the number of faulty candles her company makes.

One way to examine Ling's problem is to look at a control chart, or a graph that shows variations in quality of manufacturing over time. For example, Ling could make a control chart that looks at how many faulty candles her factory makes over the course of a week, a month, or a year. How will this help Ling? The goal of control charts is to see patterns in the data. By examining patterns, Ling can understand what's going wrong to cause the faulty candles. To help Ling out, let's look at two common patterns in control charts (cycles and trends) and what they each mean.

Cycles

Ling has a control chart that shows the proportion of faulty candles that have come off her assembly line over the last three months. She can see from day to day what percentage of the candles is not up to her quality standard. When she looks at the chart, she notices something: every week there is a similar pattern. Early in the week, there are very few faulty candles being made, but by the end of the week the number has risen dramatically. The following week, at the beginning of the work week the number goes back down, only to rise again by the end of the week.

A systematic cycle on a control chart looks like a regular up and down pattern. In this case, Ling has noticed the systematic cycle in that her assembly line is producing more faulty candles at the end of every week than at the beginning. A systematic cycle indicates that there is something that regularly (or systematically) influences production. For example, perhaps there are more faulty candles at the end of the week because Ling's workers are more tired. At the beginning of the week, they are energized and produce fewer faulty candles.

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