Table 3
The analysis of the t-test for means of DEF failures in 2010-2014 for 24 h+, 24 h−, 48 h+, 48 h−, 72 h+ and 72 h−, where W is the value of Shapiro – Wilk test, F the value of Levene test*.
| Variable | Normality | Homogeneity of variance | n | Mean | t-test |
|---|---|---|---|---|---|
| Mean 24 h+ | W = 0.94 | 18 | 10.66 | ||
| p = 0.33 | F = 3.74 | t = 3.42 | |||
| Mean 24 h− | W = 0.89 | p = 0.0095 | 18 | 4.03 | p = 0.002 |
| p = 0.04 | |||||
| Mean 48 h+ | W = 0.90 | 18 | 9.17 | ||
| p = 0.054 | F = 2.26 | t = 2.21 | |||
| Mean 48 h− | W = 0.96 | p = 0.10 | 18 | 5.91 | p = 0.03 |
| p = 0.63 | |||||
| Mean 72 h+ | W = 0.92 | 14 | 7.23 | ||
| p = 0.22 | F = 2.22 | t = 2.13 | |||
| Mean 72 h− | W = 0.92 | p = 0.16 | 14 | 4.31 | p = 0.04 |
| p = 0.23 |
In the case of the assumption of normality of our distributions, we formulate hypothesis H0: the distribution of data is normal for mean 24 h+ as for mean 24 h−. Using the Shapiro-Wilk test (for a small sample) we observe that the assumption on normality is not fulfilled for mean 24 h−, because p = 0.04, < 0.05. Thus, we reject H0 and accept H1, which says that the distribution of data is not normal. Analogically, we check assumptions for the Levene test.
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