First Response Data Analysis Tool
Once you have your data, it's easy to spin your wheels trying to figure out what to do next. Graphing your data right away can give you valuable insight and direction. Minitab's new individual value plot allows you to view important data features, find miscoded values, and identify unusual cases.
Creating and reviewing an individual value plot first can help you set the appropriate course for your analysis and avoid wasted time and frustration.
How Can Individual Value Plots Help Me?
Identify individual values
Unlike boxplots, individual value plots display all data values. If you typically use boxplots to evaluate your data, note that individual value plots may be more informative for a small amount of data.
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The boxplots for the two variables look identical, but...
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The individual value plot of the same data shows that there are many more values for Batch 1 than for Batch 2. (Blue dots represent the median.)
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You can use individual value plots to identify possible outliers and other values of interest. Hover the cursor over any point to see its exact value and position in the worksheet.
Individual value plots can also clearly illustrate characteristics of the data distribution. In this graph, most values are clustered between 4 and 10, but a few points are outside this range. The sample mean appears to be about 5.
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Minitab jitters (randomly nudges) the points horizontally, so that one value doesn't obscure another. You can edit the plot to turn off jitter. |
Make group comparisons
Because they display all values for all groups at the same time, individual value plots are especially helpful when comparing variables, groups, and even subgroups.
This plot shows the diameter of pipes produced on two lines over four shifts. You can see that the diameters of pipes produced by Line 1 seem to increase in variability across shifts, while the diameters of pipes produced by Line 2 appear more stable. (Blue dots represent the mean.)
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Support other analyses
An individual value plot is one of the built-in graphs available with many Minitab statistical analyses. You can easily display an individual value plot while you perform these analyses. In the analysis dialog box, simply click Graphs and check Individual Value Plot.
Some built-in individual value plots include specific analysis information. For example, the plot that accompanies a 1-sample t-test displays the 95% confidence interval for the mean and the reference value for the null hypothesis mean. These plots give you a graphical representation of the analysis results.
This plot accompanies a 1-sample t-test. All the data values are between 4.5 and 5.75. The reference mean lies outside of the confidence interval, suggesting that the mean differs from the hypothesized value.
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In some built-in individual value plots, data are arranged horizontally.
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Individual Value Plot: a Case Study
Suppose salad dressing is bottled using four different filling machines, and you want to make sure that the bottles are filled correctly to 16 ounces. You weigh 30 samples from each machine. You plan to run an ANOVA to see if the means of the samples from each machine are equal. But first, you display an individual value plot of the samples to get a better "feel" for the data.
- Choose Graph > Individual Value Plots.
- Under One Y, choose With Groups. Click OK.
- In Graph variables, enter Weight.
- In Categorical variables for grouping, enter Machine.
- Click Data View.
- Under Data Display, check Interval bar and Mean symbol.
- Click OK in each dialog box.
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The mean fill weight is about 16 ounces for Fill2, Fill3, and Fill4, with no suspicious data points. For Fill1, however, the mean appears higher, with a possible outlier at the lower end.
Before you continue with the analysis, you may want to investigate problems with the Fill1 machine.
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Putting Individual Value Plots to Use
Use Minitab's individual value plot to get a quick overview of your data before you begin your analysis—especially if you have a small data set or you want to compare groups. The insight you gain can help you decide what to do next, and may save you time spent needlessly exploring other paths. For more information on individual value plots and other Minitab graphs, see Minitab Help.