Databases Reference
In-Depth Information
Figure 7.14: Moving the representative points 20% of the distance to the clus-
ter's centroid
much smaller, it might well be argued that combining the points of the ring and
circle into a single cluster reflected the true state of affairs. For instance, the
rings of Saturn have narrow gaps between them, but it is reasonable to visualize
the rings as a single object, rather than several concentric objects. In the case
of Fig. 7.14 the choice of
1. The fraction of the distance to the centroid that we move the representa-
tive points and
2. The choice of how far apart representative points of two clusters need to
be to avoid merger
together determine whether we regard Fig. 7.12 as one cluster or two.
2
The last step of CURE is point assignment. Each point p is brought from
secondary storage and compared with the representative points. We assign p
to the cluster of the representative point that is closest to p.
Example 7.13 : In our running example, points within the ring will surely
be closer to one of the ring's representative points than to any representative
point of the circle. Likewise, points within the circle will surely be closest to a
representative point of the circle. An outlier - a point not within the ring or
the circle - will be assigned to the ring if it is outside the ring. If the outlier is
between the ring and the circle, it will be assigned to one or the other, somewhat
favoring the ring because its representative points have been moved toward the
circle.
2
7.4.3 Exercises for Section 7.4
Exercise 7.4.1 : Consider two clusters that are a circle and a surrounding ring,
as in the running example of this section. Suppose:
