What does logp output in classify exactly mean?

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Maria
Maria on 28 Apr 2014
Commented: Maria on 6 May 2014
Hi all!
I am using the classify function but I obtain positive values in the logp output parameter. If I understand correctly this is the logarithm of a probability and consequently should't be larger than 0. Is that correct? If so, what could cause getting these values?
Thank you very much!
  4 Comments
Maria
Maria on 28 Apr 2014
Let's say that our query is:
query = [-0.6824 -0.0764 -0.4608 -0.0770 -0.5227]
Our training data is: train_x =
[-0.6837 -0.0789 -0.5838 -0.0436 -0.6582;
-0.5692 -0.0707 -0.5459 -0.0083 -0.5791;
-0.6475 -0.0597 -0.6075 -0.1157 -0.6768;
-0.7199 -0.0655 -0.5886 -0.1927 -0.6442;
-0.8650 -0.0616 -0.3579 -0.0563 -0.4931;
-0.7285 -0.0545 -0.2680 -0.1328 -0.3348;
-0.7717 -0.0749 -0.6171 -0.1440 -0.7033;
-0.4889 -0.0675 -0.5421 -0.1596 -0.5656;
-0.5019 -0.0822 -0.5932 -0.1313 -0.6452;
-0.5383 -0.0781 -0.6051 0.0638 -0.6635;
-0.8107 -0.0592 -0.5815 -0.2463 -0.6475;
-0.8576 -0.0607 -0.5961 -0.1486 -0.6813;
-0.8214 -0.0753 -0.6193 0.0215 -0.7097;
-0.7035 -0.0489 -0.4232 0.1721 -0.4677;
-0.8102 -0.0533 -0.2051 -0.2215 -0.3409]
and the labels : y = [-1; -1 ; -1; 1; -1; -1; -1; -1; 1; -1; -1; -1; -1; -1; -1]
Then if we do: [pred_query_fisher, train_err, pos_query, logp] = classify(query, train_x, train_y)
logp will be 7.2821
Thanks!

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Answers (1)

Ilya
Ilya on 30 Apr 2014
Quoting from the doc for classify:
[class,err,POSTERIOR,logp] = classify(...) also returns a vector logp containing estimates of the logarithms of the unconditional predictive probability density of the sample observations...
Values of probability density do not need to be less than one.
  5 Comments
Maria
Maria on 6 May 2014
I just want to classify one query at a time. I guess this output is useless in my case. Thanks

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