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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