Davies Bouldin index returns negative values ??

MarcosRLMarcosRL Member Posts: 53 Contributor II
edited November 2018 in Help
hello friends community
the metrics applying validation of clusters is obtained and the Davies Bouldin index negative returns me this is a bug?
regards

Answers

  • wesselwessel Member Posts: 537 Guru
    As far as I understand, this is not a bug.
    Its just easier to use the negative value for optimization.
  • MarcosRLMarcosRL Member Posts: 53 Contributor II
    excuse my ignorance  :(, but I do not understand  ???.
    In theory lei Davies Boulding index is a number greater than zero.
    How's that for optimization you mention?
    regards
  • Stefan_acStefan_ac Member Posts: 2 Contributor I

    Hey guys,

    i am searching for an explaxation to this negative mentioned davies-bouldin values. Please, can anyone explain to me why Rapidminer ist calculation negative values?

     

    My Performance Vectors are looking like this:

    Performance Vector

    Average within centroid distance

    cluster_0: -1.831
    cluster_1: -1.931
    cluster_2: -1.856
    cluster_3: -1.897
    cluster_4: -1.903
    cluster_5: -1.885
    cluster_6: -1.891
    cluster_7: -1.878
    cluster_8: -1.818
    cluster_9: -1.869

    Davides Bouldin: -1.974

     

    Thanks in advance for your help and reply,

    Stefan

  • MartinLiebigMartinLiebig Administrator, Moderator, Employee, RapidMiner Certified Analyst, RapidMiner Certified Expert, University Professor Posts: 3,453 RM Data Scientist

    Hi Stefan,

     

    i do not know why, but by default the values are multiplied by -1 so that you can run a minimizer on it. That's why the operator has an option called maximize with this description:

    maximize

    Description: This parameter specifies if the results should be maximized. If set to true, the result is not multiplied by minus one.

     

    Simply check it and get what you like more :)

    Best,

    Martin

    - Sr. Director Data Solutions, Altair RapidMiner -
    Dortmund, Germany
  • Stefan_acStefan_ac Member Posts: 2 Contributor I

    Hey Martin, 

    thanks for your fast reply.

    I read about the multiplication by -1. Thanks for the advanced paramrter advice. Now my values turn in positive ones. BUT, I am still wondering why the values are greater >1. Usualy Davies Boulding values are between 0 and 1 (0="good" clusters and 1="bad" clusters). Now that my values are greater 1, do you have a suggestions for interpretation?

    Regards,

    Stefan

  • MartinLiebigMartinLiebig Administrator, Moderator, Employee, RapidMiner Certified Analyst, RapidMiner Certified Expert, University Professor Posts: 3,453 RM Data Scientist

    Hi Stefan,

     

    why do you think this should be normalized? According to Wikipedia: https://en.wikipedia.org/wiki/Davies%E2%80%93Bouldin_index i don't see any reason to have it in [0,1].

     

    Nevertheless you can of course normalize the DB index.

     

    ~Martin

    - Sr. Director Data Solutions, Altair RapidMiner -
    Dortmund, Germany
  • alle0908alle0908 Member Posts: 1 Contributor I

    So in either case, does the most optimal cluster according to DB index have the resulting DB that is farthest from zero, or closest? Or asked another way, for the absolute value of DB is a DB index of 10 better or worse than a DB index of 1?

  • MartinLiebigMartinLiebig Administrator, Moderator, Employee, RapidMiner Certified Analyst, RapidMiner Certified Expert, University Professor Posts: 3,453 RM Data Scientist

    Hi,

     

    as far as i know smaller absolute values are better. From the doc:

     

    davies_bouldin: The algorithms that produce clusters with low intra-cluster distances (high intra-cluster similarity) and high inter-cluster distances (low inter-cluster similarity) will have a low Davies–Bouldin index, the clustering algorithm that produces a collection of clusters with the smallest Davies–Bouldin index is considered the best algorithm based on this criterion.

     

    Best,

    Martin

    - Sr. Director Data Solutions, Altair RapidMiner -
    Dortmund, Germany
    Thomas_OttPavithra_Rao
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