R help - Feb 2016 - How a clustering algorithm in R can end up with negative silhouette values?

Hi,


We know that clustering methods in R assign observations to the closest medoids.
Hence, it is supposed to be the closest cluster each observation can have. So, I
wonder how it is possible to have negative values of silhouette , while we are
supposedly assign each observation to the closest cluster and the formula in
silhouette method cannot get negative?


Behnam.

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That means that points have been assigned to the wrong groups. This
may readily happen with a clustering method like cluster::clara() that
uses a subset of the data to cluster a dataset too large to analyze as
a unit. Negative silhouette numbers strongly suggest that your
clustering parameters should be changed.

Sarah

On Fri, Feb 19, 2016 at 6:33 AM, ABABAEI, Behnam
<Behnam.ABABAEI at limagrain.com> wrote:
> Hi,
>
>
> We know that clustering methods in R assign observations to the closest
medoids. Hence, it is supposed to be the closest cluster each observation can
have. So, I wonder how it is possible to have negative values of silhouette ,
while we are supposedly assign each observation to the closest cluster and the
formula in silhouette method cannot get negative?
>
>
> Behnam.
>

Hi Sarah,

Thank you for the response. But it is said in its description that after each
run (sample), each observation in the whole dataset is assigned to the closest
cluster. So how is it possible for one observation to be wrongly allocated, even
with clara?

Behnam

Behnam



On Fri, Feb 19, 2016 at 11:48 AM -0800, "Sarah Goslee"
<sarah.goslee at gmail.com<mailto:sarah.goslee at gmail.com>> wrote:

That means that points have been assigned to the wrong groups. This
may readily happen with a clustering method like cluster::clara() that
uses a subset of the data to cluster a dataset too large to analyze as
a unit. Negative silhouette numbers strongly suggest that your
clustering parameters should be changed.

Sarah

On Fri, Feb 19, 2016 at 6:33 AM, ABABAEI, Behnam
<Behnam.ABABAEI at limagrain.com> wrote:
> Hi,
>
>
> We know that clustering methods in R assign observations to the closest
medoids. Hence, it is supposed to be the closest cluster each observation can
have. So, I wonder how it is possible to have negative values of silhouette ,
while we are supposedly assign each observation to the closest cluster and the
formula in silhouette method cannot get negative?
>
>
> Behnam.
>
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