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When I perform cluster analysis in SAS, the SAS log sometimes return a warning something like this:

    WARNING: Ties for minimum distance between clusters have been 
             detected at 4 level(s) in the cluster history.

Depends on the clustering methods (all hierarchical clustering methods) I choose, the number of ties will change. But regardless of the methods, I want to know if there is a solution to this issue. Or is this OK to ignore since the issue arises from the fact that the distance of values are really close to each other?

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Ties in hierarchical clustering are common, in particular when you have discrete data, duplicate records and such. If you run hierarchical clustering with Manhattan distance on the classic iris data set, you will have plenty of ties. Real data just is not a smooth as artificially generated data with 20+ digits of precision. Real data just has ties.

I do not understand why SAS considers this to be worth a warning. But SAS clustering capabilities are very outdated anyway.

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  • $\begingroup$ I've been ignoring the warning so far but it's really bugging me to see that exclamation mark every time I run PROC CLUSTER. $\endgroup$
    – Ken
    May 8, 2012 at 19:19
  • $\begingroup$ Maybe they are just scared that all your objects might be tied, at which point the result will be totally worthless. I don't know. Maybe ask in some SAS expert list. $\endgroup$ May 8, 2012 at 19:23

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