Example of an interval plot

A chemical engineer assesses the quality of the plastic that is used in manufacturing billiard balls. Because higher elasticity is better, the engineer wants to determine whether either of two additives improves the elasticity of the plastic. The engineer prepares 10 samples with each of the two additives and 10 samples with no additive and measures the elasticity of each.

As part of the initial investigation, the engineer creates an individual value plot to compare the elasticity of the samples. The engineer adds mean symbols, confidence intervals, and mean connect lines to the plot to compare the differences between the group means. (A plot with confidence intervals is sometimes called an interval plot.)

  1. Open the sample data, BilliardBallElasticity.MTW.
  2. Choose Graphs > Individual Value Plot > Single Y Variable: With Groups.
  3. In Variable, enter Elastic.
  4. In Group Variable, enter Additive.
  5. Click OK.
  6. Click the graph to select it, then click the plus sign beside the graph to open the Graph Elements menu.
  7. Click the triangle next to Data Display ( ), then do the following:
    1. Select 95% Mean Confidence Interval Bar.
    2. Select Mean Symbol.
    3. Select Mean Connect Line.
    4. Deselect Individual Symbols.

Interpret the results

Additive 1 appears to increase elasticity. Additive 2 may actually decrease elasticity. For samples that have no additive (0), the elasticity appears to be between additives 1 and 2. The variation within each group appears to be about the same.

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