29.5 $P$-values: Mean differences

A $P$-value determines if the sample data are consistent with the assumption (Table 28.1), Since $t=1.681$, the one-tailed $P$-value is between 2.5% and 16% based on the 68–95–99.7 rule. This is a wide, and inconclusive, interval. Software gives a more precise $P$-value (jamovi: Fig. 29.4; SPSS: Fig. 29.5): the two-tailed $P$-value is $0.127$, so the one-tailed $P$-value is $0.127/2 = 0.0635$.

FIGURE 29.4: jamovi output for the insulation data

FIGURE 29.5: SPSS output for the insulation data

The software clarifies how the differences have been computed:

• jamovi: At the left of the output (Fig. 29.4), the order implies the differences are found as Before minus After.

• SPSS: At the left of the output (Fig. 29.5), the difference is described as Before ${}-{}$ After.

Click on the hotspots in the following image, to see how to read the SPSS output.

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Peter K. Dunn, 2021: CC BY-NC-SA 4.0