In the manufacturing of a plastic material, it is believed that the cooling time has an influence on the impact strength. Therefore a study is carried out in which plastic material impact strength is determined for 4 different cooling times. The results of this experiment are shown in the following table: Cooling times in seconds (x) 15 25 35 40 Impact strength in kJ/m (y) 42.1 36.0 31.8 28.7 The following statistics may be used: t = 28.75, y = 34.65, Sxx = 368.75