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Columns to be tested must be defined in groups of four. If there are more than four categories the net difference test is carried out on the preceding four category elements nearest the net difference element. If there are fewer than four categories, the net difference test is not carried out. The net difference test is not available across the different sections of a table that are created by nesting and concatenation.
When you specify a net difference test, you must also add a net difference element to the top of the table. If the table is nested, this must be in the innermost nesting level.