What is the meaning of jitter in visualize tab of weka

Question

In weka I load an arff file. I can view the relationship between attributes using the visualize tab.

However I can't understand the meaning of the jitter slider. What is its purpose?

Sediment

Answer 1

You can find the answer in the mailing list archives:

The jitter function in the Visualize panel just adds artificial random noise to the coordinates of the plotted points in order to spread the data out a bit (so that you can see points that might have been obscured by others).

Perea

Load the contact lenses example from Weka, and visualize it. In the first plot, some points are exactly on top of each other. By adding jitter, you can see how many points there are for each combination. This won't work that good for larger data sets, but you can still get an impression. For discrete values, this is quite important. – Elocution

Answer 2

I don't know weka, but generally jitter is a term for the variation of a periodic signal to some reference interval. I'm guessing the slider allows you to set some range or threshold below which data points are treated as being regular, or to modify the output to introduce some variation. The wikipedia entry can give you some background.

Update: from this pdf, the jitter slider is for this purpose:

“Jitter” option to deal with nominal attributes (and to detect “hidden”data points)

Based on the accompanying slide it looks like it introduces some variation in the visualisation, perhaps to show when two data points overlap.

Update 2: This google books extract (to Data mining By Ian H. Witten, Eibe Frank) seems to confirm my guess:

[jitter] is a random displacement applied to X and Y values to separate points that lie on top of one another. Without jitter, 1000 instances at the same data point would look just the same as 1 instance

Flotation

Answer 3

I don't know the products you mention, but jittering generally means randomising the sample positions. Eg, in ray tracing you would normally render a ray though each pixel on the screen. Jittering adds a random offset to each ray to reduce issues caused by regular aliasing.

Hettie