问题
I would like to calculate entropy of this example scheme
http://nlp.stanford.edu/IR-book/html/htmledition/evaluation-of-clustering-1.html
Can anybody please explain step by step with real values? I know there are unliminted number of formulas but i am really bad at understanding formulas :)
For example in the given image, how to calculate purity is clearly and well explained
The question is very clear. I need an example how to calculate entropy of this clustering scheme. It can be step by step explanation. It can be C# code or Phyton Code to calculate such scheme
Here entropy formula
I will code this in C#
Thank you very much for any help
I need answer like given in here : https://stats.stackexchange.com/questions/95731/how-to-calculate-purity
回答1:
This section of the NLP book is a little confusing I will admit because they don't follow through with the complete calculation of the external measure of cluster entropy, instead they focus on the calculation of an individual cluster entropy calculation. Instead I will try to use a more intuitive set of variables and include the complete method for calculating the external measure of total entropy.
The total entropy of a clustering is:
where:
is the set of clusters
H(w) is a single clusters entropy
N_w is the number of points in cluster w
N is the total number of points.
Entropy of a cluster w
where: c is a classification in the set C of all classifications
P(w_c) is probability of a data point being classified as c in cluster w.
To make this usable we can substitute the probability with the MLE (maximum likelihood estimate) of this probability to arrive at:
where:
|w_c| is the count of points classified as c in cluster w
n_w is the count of points in cluster w
So in the example given you have 3 clusters (w_1,w_2,w_3), and we will calculate the entropy for each cluster separately, for each of the 3 classifications (x,circle,diamond).
H(w_1) = (5/6)log_2(5/6) + (1/6)log_2(1/6) + (0/6)log_2(0/6) = -.650
H(w_2) = (1/6)log_2(1/6) + (4/6)log_2(4/6) + (1/6)log_2(1/6) = -1.252
H(w_3) = (2/5)log_2(2/5) + (0/5)log_2(0/5) + (3/5)log_2(3/5) = -.971
So then to find the total entropy for a set of clusters, you take the sum of the entropies times the relative weight of each cluster.
H(Omega) = (-.650 * 6/17) + (-1.252 * 6/17) + (-.971 * 5/17)
H(Omega) = -.956
I hope this helps, please feel free to verify and provide feedback.
回答2:
The computation is straightforward.
The probabilities are NumberOfMatches/NumberOfCandidates.
The you apply base2 logarithms and take the sums.
Usually, you will weight the clusters by their relative sizes.
The only thing to pay attention to is when p=0. Then the logarithm is undefined. But we can safely use p log p = 0 if p = 0 because of the p outside the logarithm.
Because log 1 = 0 the minimum entropy is 0. Perfect results must score entropy 0, or you have an error.
来源：https://stackoverflow.com/questions/35709562/how-to-calculate-clustering-entropy-a-working-example-or-software-code