# Understanding interaction between two variables

I am wondering how do I interpret the third (interaction plot) in the below figure which come from this link http://www.milbo.org/doc/plotmo-notes.pdf

What does it imply? What are the axis of temp and humidity in the third plot? Do they go from high to low or low to high?

This is just a 3-dimensional plot of ozone vs temp and humidity. In this plot, you can see that the effect of humidity on ozone

• is quite small (almost flat) for low temperatures
• is quite big (very steep) for high temperatures,

thus indicating the presence of an interaction between humidity and temperature, a fact that cannot be seen from the 2-dimensional plots.

As indicated by @mkt in his comment below, the 3D plot should indicate which side of the axes is low and which side is high, but it can be inferred from the patterns in the 2D plots.

• Thank you. My question was how did you know which side of the temperature axis is low temperature and which side is high temperature? – 89_Simple Apr 16 '18 at 13:36
• @Crop89 I agree it's a bit confusing. Ideally, the 3D plot should indicate this, but it can be inferred from the patterns in the 2D plots. – mkt - Reinstate Monica Apr 16 '18 at 13:42
• @Crop89: I agree with mkt; see edit. – ocram Apr 16 '18 at 13:47
• I updated the document referred to by the original poster: I added better labels to the interaction plot. – Stephen Milborrow May 15 '18 at 17:47