Base for T statistics
Quick reference
The default value for a small base, when the base is flagged as small, is 100 and the default value for a very small base, when no statistics are calculated, is 30. To define your own small base figure, place the keyword:
smallbase=number
on the a, sectbeg, flt or tab statement or on the tstat statement for the test. To define your own very small base figure, place the keyword:
minbase=number
on the a, sectbeg, flt or tab statement or on the tstat statement for the test.
More information
The base element plays an important part in T statistics, not least because its size determines whether or not the tests are run at all.
The previous section explained how Quantum uses the effective base rather than the weighted base in the calculation of T statistics in weighted tables. In unweighted tables the unweighted base is the same as the effective base.
Where an axis contains more than one base element, the T statistic is calculated on the most recent base. In weighted runs, a new nsw element is created for each base with the same conditions as the main base.
All statistics are more reliable if they are based on large samples. If the base for a T statistic in an unweighted table, or the effective base in a weighted table, is less than 100, Quantum treats it as a small base and flags the base figure for the column in the printed table with a single asterisk.
If the unweighted base/effective base for a column (row for the Paired Preference test) is less than 30, Quantum sees it as a very small base and flags it with two asterisks and does not carry out the test. Quantum issues the message ‘base too small (<30)’ to warn you that a test has been suppressed.
You can specify your own values to act as small and very small bases instead of these defaults. To define your own small base, place the option:
smallbase=number
on the a, sectbeg, flt or tab statement or on the tstat statement that requests the test.
Note The maximum value for both a small base and a very small base is 255, and the minimum value for both is zero.
To define your own value for a very small base, place the option:
minbase=number
on the a, sectbeg, flt or tab statement or the tstat statement of the test itself.
Note It is possible to request multiple statistical tests on a single table and to define different small and very small bases on each test. However, when you do this Quantum does not use the small and very small bases defined for each test. Instead, Quantum searches the tests in a specific sequence and uses for all the tests on the table the small and very small base figures that are in force on the first test it finds. If minbase and smallbase were not defined on the test selected by Quantum, the default values are used. The sequence for the search is paired preference test, significant net difference test, T-test on column proportions, T-test on column means and Newman-Keuls test on means.
See also