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Industrial Statistics with Minitab by Xavier Tort-Martorell Llabres, Lluis Marco Almagro, Pere Grima Cintas

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12.5 U Piece

A mechanism comprising a U-shaped part is manufactured. Its critical dimension is the width of the mouth, X. These parts, before being assembled in the mechanism, are subjected to a heat treatment in a furnace to increase its hardness. The oven is similar to a domestic oven (perhaps slightly higher) and the parts are placed in 5 trays, one above the other, supported in a two side support.
The nominal value of X is 12 ± 0.1 mm. We want to know if this heat treatment produces a systematic change of this value. To conduct the study the value of X before and after the treatment is measured for 100 parts. The number of the tray where each part has been placed is also registered.
The data are in the file U_PIECE.MTW. What conclusions can be drawn?

To get a first idea we create a new column with the difference: After-Before and we draw a dotplot:

Graph > Dotplot > Simple

Notice that 0 appears off-centred and in most cases the heat treatment increases the value of this dimension. To confirm the influence we perform a comparison of means test for paired data:

Stat > Basic Statistics > Paired t,

First sample: Before, Second Sample: After

The heat treatment increases the X dimension 0.03 mm on average and this difference is clearly significant. At a first sight, it ...

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