Jittered Density Plot

A jittered density plot is a simple graph that illustrates the distribution of a number of data points along a dimension. The x-axis of the plot provides the dimension of interest. The y-axis is a randomly assigned number that "jitters" plotted points to reduce overlap. A point is plotted so that it lies above the value in the x-dimension that it is associated with, at some randomly selected height in the y-dimension.

Jittered density plots have been very useful in the interpretation of the internal structure of multilayer perceptrons that are composed of value units. It was discovered that jittered density plots of hidden unit activities (where each point in the plot corresponds to a pattern presented to the network) were often highly structured, organizing themselves into distinct bands (Berkeley et al, 1995). Analysis of shared properties of the patterns that fell into the same band revealed interpretable features that permit the internal algorithm of the network to be deciphered (e.g. Dawson et al., 2000).

References:

1. Berkeley, I. S. N., Dawson, M. R. W., Medler, D. A., Schopflocher, D. P., & Hornsby, L. (1995). Density plots of hidden value unit activations reveal interpretable bands. Connection Science, 7, 167-186.
2. Dawson, M. R. W., Medler, D. A., McCaughan, D. B., Willson, L., & Carbonaro, M. (2000). Using extra output learning to insert a symbolic theory into a connectionist network. Minds And Machines, 10, 171-201.

(Added March 2010)