As the line chart shows, the yields have quite different locations. While this is unimportant to the processor, the Kolmogorov-Smirnov test is sensitive to differences in location and shape. Centering each yield around its mean will eliminate that difference and allow for a comparison of their shapes.

To center Aflatoxin PPB, from the Data Editor menus choose:

Transform
Compute...

Type ctrtoxin as the target variable.

Click Type&Label.

Type Centered aflatoxin level as the label.

Click Continue.

Click If in the Compute Variable dialog box.

Select Include if case satisfies condition.

Type yield=4 in the expression area.

Click Continue.

The mean aflatoxin level for yield 4 is 14.6875, so type toxin - 14.6875 in the expression area.

Click OK.

That takes care of yield 4, but yield 8 remains.

Recall the Compute Variable dialog box.

Click If in the Compute Variable dialog box.

Type yield=8 in the expression area.

Click Continue.

Type toxin - 8.4375 in the expression area.

Click OK.

Click OK to finish centering the variable.