Figure 2: Testing the influence of initial value of PD on its progression. The regression line has a growing trend at the correlation coefficient r = 0.2901 (p = 0.0019). It is therefore, statistically significant direct dependence of the progression on the initial value of PD. Thus, the higher the initial PD, the greater is the value of progression. The value of the correlation coefficient r = 0.2901, however, shows rather weak dependence.