To print an effective base which will not affect the count in any c=– elements, type:

To print an effective base which will affect the count in any c=– elements, type:

on an n01, n10, n11 or n15 statement.

Quantum creates T statistics on weighted tables using a special base figure called the effective base. The purpose of the effective base is to reduce the likelihood of the statistics producing significant results because the weighting has made adjustments to the data.

When surveys are conducted it is impossible to interview everyone, so what usually happens is that a sample of respondents is interviewed and then the results are weighted so that they match the total population or the proportions in the total population. As an illustration, consider the case where the data is weighted by sex. The sample consists of 30% women and 70% men whereas the total population is made up of 52% women and 48% men.

For more information on weighting, see Weighting.

In this case, the weighting inflates the answers given by women and deflates the answers given by men in order to match the population proportions. Answers given by women count as greater than 1 in the tables; answers given by men count as less than 1. To be precise, women’s answers count as 52/30=1.733, and men’s answers count as 48/70=0.686.

The effective base takes these adjustments into account. It is calculated by dividing the squared sum of weighting factors for an axis by the sum of the squared weighting factors; that is:

If the data for a particular column has both unweighted and weighted bases of 40, and comes from 12 women and 28 men, the effective base is 32.509. The calculation that produces this value is:

The effective base is a good criterion for judging how good your weighting is. If the weighting is inflating the answers from a particular group by a large factor, the effective base tends to be much smaller than the unweighted and weighted bases. The closer the effective base is to the unweighted base, the better the weighting is.

You can print the effective base in your tables in two ways; one will affect the results of any special c=– elements and the other will not. c=– conditions can be used to produce a count of respondents who have not been included in any element since the last base in the axis. When making this calculation, Quantum ignores all respondents in the last base element, but includes all respondents in an effective base element created using the effbase keyword. This could result in no one appearing in the c=– element, thus defeating its purpose.

To print the effective base without affecting any c=– results, write:

For example:

Otherwise, use the effbase keyword on an n10, n11, n01 or n15 statement as follows:

For more information on when to use n31 and when to use effbase, see Printing the effective base.

In order for Quantum to report the effective base correctly, make sure that your axis is specified as follows: