Belinda Chiera - 5 months ago 84

R Question

I've been searching for an answer for this question for quite a while, so I'm hoping someone can help me. I'm using dbscan from the fpc library in R. For example, I am looking at the USArrests data set and am using dbscan on it as follows:

`library(fpc)`

ds <- dbscan(USArrests,eps=20)

Choosing eps was merely by trial and error in this case. However I am wondering if there is a function or code available to automate the choice of the best eps/minpts. I know some books recommend producing a plot of the kth sorted distance to its nearest neighbour. That is, the x-axis represents "Points sorted according to distance to kth nearest neighbour" and the y-axis represents the "kth nearest neighbour distance".

This type of plot is useful for helping choose an appropriate value for eps and minpts. I hope I have provided enough information for someone to be help me out. I wanted to post a pic of what I meant however I'm still a newbie so can't post an image just yet.

Answer

There is no general way of choosing minPts. It depends on what *you* want to find. A low minPts means it will build more clusters from noise, so don't choose it too small.

For epsilon, there are various aspects. It again boils down to choosing whatever works on *this* data set and *this* minPts and *this* distance function and *this* normalization. You can try to do a knn distance histogram and choose a "knee" there, but there might be no visible one, or multiple.

OPTICS is a successor to DBSCAN that does not need the epsilon parameter (except for performance reasons with index support, see Wikipedia). It's much nicer, but I believe it is a pain to implement in R, because it needs advanced data structures (ideally, a data index tree for acceleration and an *updatable* heap for the priority queue), and R is all about matrix operations.

Naively, one can imagine OPTICS as doing all values of Epsilon at the same time, and putting the results in a cluster hierarchy.

The first thing you need to check however - pretty much independent of whatever clustering algorithm you are going to use - is to make sure you have a useful distance function and appropriate data normalization. If your distance degenerates, *no* clustering algorithm will work.