|
Cross Tabulation
Cross tabulation is a very basic and simple form of data analysis, well known in statistics, and widely used for reporting. A two dimensional cross-tab is similar to a spreadsheet, with both row and column headings as attribute values. The cells in the spreadsheet represent an aggregate operation, usually the number of co-occurances of the attribute values together. Many cross-tabs are effectively equivalent to a 3D bar graph which displays co-occurence counts.
Consider the table in the previous section. A cross-tab for the profit level could look as follows:
| |
CA |
AZ |
NY |
Blue |
Green |
Red |
|
Profit High |
1 |
0 |
1 |
2 |
0 |
0 |
|
Profit Avg |
1 |
0 |
1 |
0 |
1 |
1 |
|
Profit Low |
0 |
2 |
0 |
0 |
1 |
1 |
|
Here we have not included the fields Manufacturer and City because the cross-tab would look too large. However, as is readily seen here, the fact that the count of co-occurence of Blue and High is above the others indicates a stronger relationship.
When dealing with a small number of non-numeric values, cross-tabs are simple enough to use and find some conditional logic relationships (but not attribute logic, affinities or other forms of logic). Cross-tabs usually run into four classes of problems: first when the number of non-numeric values goes up, second when one has to deal with numeric values, third when several conjunctions are involved, and fourth when the relationships are not just based on counts.
Agents and belief networks are variations on the cross-tab theme and will be discussed next.
Copyright (C) 1997, Journal of Data Warehousing, December 1997 |
