General notes
When using Quantum statistics, remember these points:
▪Many of the statistics require that the axis (or axes, with table-level statistics) contains one or more base elements. In the case of some axis-level tests, these are only necessary to separate segments of an axis from one another. Whether or not a base element is required is defined in the notes which follow the description of each statistical test.
Base elements can be printing or non-printing elements created using n10 or n11 statements, or base options on n01, n15, col or val statements.
▪Many of the statistics require a certain number of basic count (totalizable) rows. These are rows created by n01, n15, col or val statements which obtain information directly from the data file.
In an ordinary job, these rows are generally counts of people.
In levels (hierarchical or trailer card) jobs, they are counts of households, people, trips made, and so on. Other elements, such as text-only elements, are ignored.
▪Some tests (generally those based on table-level statistics) require that the elements to be tested are mutually exclusive. This means that respondents can be present in at most one element of the axis. Examples of mutually exclusive axes are sex, age, marital status, product preferred, and so on, where someone present in one category will not be present in any other.
Complex axes often contain elements which are not wholly mutually exclusive: for example, one containing elements for sex and elements for age, where respondents may be present in both a sex category and an age category. Axes of this type may be used as the basis for statistics although in many cases the values for the overlapping categories should be ignored.
▪Some statistical tests become unreliable when performed on tables containing cells with small numbers of respondents in them, for example, less than 10. These are generally the tests resulting in a chi-squared or Z statistic. In these cases it is best, if possible, to combine the row or column element, or both, with the logically nearest element to increase the cell sizes. Otherwise, exclude the row or column from the test altogether by specifying it with the option ntot. (The logically nearest element is the one whose meaning is nearest to that of the small element: for example, the ‘18–24’ age range would be combined with the ‘25–34’ age range rather than the ‘55 and over’ age range.)
See also