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Histogram: How to do itThe Quality Toolbook > Histogram > How to do it When to use it | How to understand it | Example | How to do it | Practical variations
How to do it
If the data may be stratified (for example, by time, people or material), ensure enough measurements are taken to build a Histogram for each group. For example, if there are three groups, then take at least 150 measurements. (a) Decide how many bars you wish to show on the chart. This should be sufficient to show the shape of the overall frequency distribution. A rule of thumb for this is based on the number of measurements made is shown in the table below. Table 1. Estimating number of bars
(b) Identify the range of values that may be measured. This is given by the difference between the largest and the smallest probable measurement values. An expected average can help with this, as it is usually in the middle. (c) Determine the value range for each bar. This is given by the range of values from (b), divided by the number of bars from (a). Round this to the nearest sensible unit, arranging for any specification limits fall between bars, not within them. For example, 100 measurements, ranging between 10 and 50 cm are divided into 8 bars of 5 cm each. (8 bars was selected from the choice of 7 to 12 in the table as it resulted in a neat bar width of 5 cm).
Table 2. Example measurement table
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