If the T-test is to be performed on overlapping data, type:

on the a, sectbeg, flt or tab statement.

To suppress the footnote that is automatically generated whenever the overlap formulae are used, put the keyword:

on the a, sectbeg, flt or tab statement.

The T-tests on column proportions and column means, the significant net difference test and the Newman-Keuls test will work on mutually exclusive or overlapping data.

When the data is overlapping, the formulae which calculate the statistics must be modified to take this into account. To do this, place the keyword overlap on the tab statement of the table to be tested, or on the a, flt or sectbeg statement above it:

To clarify the difference between mutually exclusive data and overlapping data consider a question that asks respondents which of two products they tried. If you create an element that is ‘Tried one product only’ it is single coded so overlap is not needed. The element ‘Tried both A and B’ will always be multicoded so you must use overlap. Elements such as ‘Tried A or B, or A and B’ could be single coded if no one tried more than one product, or multicoded if some respondents tried both products. You should always use overlap with elements of this type. You should also use overlap if you are testing combinations of overlapping and non-overlapping data.

If you do not use overlap when the data is overlapping you can obtain incorrect results. Specifying overlap when the data is mutually exclusive does not affect the validity of the statistic as this part of the calculation produces a value of 0.0.

When you request tests on overlapping data, Quantum displays the message ‘Overlap formulae used’ as part of the standard footnote, just above the small base/very small base messages. You can suppress this message by placing the keyword nooverlapfoot in the a, sectbeg, flt or tab statement. To switch the message back on after switching it off use overlapfoot.

For a more on the theory of overlapping samples, see Kish, Survey Sampling.