Figure 1. Example of a heat map with a positive relation
(this site uses frames, if you do not see the weblecture and definitions frames on the right you can click here, if you don't see a menu on the left and want to go to the home page click here)
In a heatmap the color or gradient of an area indicates the quantity relative to other areas. An example is shown in Figure 1.
Figure 1. Example of a heat map with a positive relation
In the example, we see a green/yellow belt going from the top left to the bottom right. This indicates the values in the combinations where people had similar opinions on both statements were relatively high, while few people had different opinions on each of the two variables. This would suggest the opinion on one statement has an influence/relation with the other. The relation could also be the other way around (someone who fully agrees with one statement is likely to fully disagree with the other), as shown in Figure 2.
Figure 2. Example of a heat map with a negative relation
A heatmap was once tried to be trademarked by Cormac Kinney in 1991, as to be used for real-time display of financial market information. However, the idea can also be applied to non-financial information, and in particular interest here for cross tables of two ordinal variables.