[R] Negative value for adjustedRandIndex?

[Pooka]

Hello, 

I am a very new user to R so please have patience with me. :clap:

I am trying to evalute the "internal response" for a couple of different
cluster methods with the help of the AdjustedRandIndex, which is included in
the mclust package. 

However, I do get a bit puzzled when I get a negative value as the value
should be in intervall of [0,1], am I correct? Have I done something wrong?
Or should I interpret the negative value as the two methods do not group the
cases to the same cluster?

I have for example used k-means and k-median, and choosen five clusters. 
Each respondent has been labelled as either beloning to cluster 1,2,3,4 or
5. I have picked out the first 32 respondents. 

Example:


a   <- c(1,2,3,4,5,2,1,5,1,2,2,4,2,3,3,2,4,5,5,4,4,4,5,5,2,3,1,5,5,5,4,1)
b   <- c(5,5,5,2,5,3,2,2,2,5,3,4,5,5,2,5,2,3,5,5,5,3,5,3,3,2,4,5,2,2,2,3)
adjustedRandIndex (a,b)

[1] -0.01608168



Thanks in advance,

Pooka
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