When statistical tests are conducted on tables containing share statistics (e.g., % Column Share), the tests are conducted using the averages, not the share statistics. This can lead to counterintuitive results when reporting the shares. This page describes how to get Q to test differences between the shares.
Example of the problem
In the table on the left, for example, the automatic statistical testing shows that Pepsi is significantly higher amongst the heavy consumers than the light consumers, but the actual numbers on the table show the opposite conclusion. The table on the right shows the averages, from which we can see that the average is higher for Pepsi amongst Heavy consumers.
Method
The solution to the problem is to transform the data so that the Average on the table is proportional to the % Column Share:
- Compute a new question that contains the share computed for each case: Automate > Browse Online Library > Create New Variables > Case-Level Shares.
- OPTIONAL: Update the name of the new variable.
- A new table will appear in the Report tree in a folder named "Shares".
- Select the original variables in the Variables and Questions tab, right-click, and select Insert Ready-Made Formula(s) > Mathematical Function(s) by Case > Sum.
- When prompted, click No.
- Tag the newly created variable as a Weight.
- OPTIONAL: Relabel the variable to something easily understood.
- On the Outputs Tab, apply the weight to the table in the "Shares" folder.
The Average and % Column Share on the tables will now coincide, and the statistical tests now, as a consequence, can be interpreted as testing differences in % Column Share.
Next
How to Access Statistics from a Table in a Rule