*3.2. Test of Homogeneity*

To determine if there is any significant statistical difference in the regression lines derived from the different test data, an Analysis of Covariance (ANCOVA) was used [25]. The topic of interest is whether the regression lines have similar slopes. This method allows for testing the null hypothesis that the regression model parameters were derived from samples estimating populations which all had equal slopes.

For this hypothesis analysis, the confidence limit level was selected to be 90% (i.e., *Pcritical* = 0.05). The *F* statistic was calculated as 0.82 and the corresponding probability (*p*-value) was estimated to be 0.618. Since *F* statistic is less than unity and *p*-value is greater than *Pcritical*, the null hypothesis could not be rejected. It was concluded that there is no statistically significant difference between the slopes of regression lines and the test of ANCOVA confirmed that the model is not influenced by the loading ratio and loading frequencies. For more information about ANCOVA, refer to References [26,27].
