I'm using WEKA to classify a certain dataset. In the results, I'm getting a Se=49% and Sp=99%. On observing the ROC, one can see that for Se=95%, Sp=88%. My question is that is there any way to adjust the classifier parameters (if any) so that my classifier is set to have a Se=95% and Sp=88% on the average.
PS: I'm using the Random Forest classifier in which the only parameters I input are # of trees, Max Depth (=0), # of features and seed.
On playing around with the ThresholdSelector in WEKA, I've managed to achieve my objective. By using Cost/Benefit analysis curve, I could find out the threshold value to suit my needs. Setting appropriate parameters in the ThresholdSelector then gave me the tuned classifier.
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K-means clustering in sklearn, number of clusters is known in advance (it is 2).
There are multiple features. Feature values are initially without any weight assigned, i.e. they are treated equally weighted. However, task is to assign custom weights to each feature, in order to get best possible clusters separation.
How to determine optimum sample weights (sample_weight) for each feature, in order to get best possible separation of the two clusters?
If this is not possible for k-means, or for sklearn, I am interested in any alternative clustering solution, the point is that I need method of automatic determination of appropriate weights for multivariate features, in order to maximize clusters separation.
In meantime, I have implemented following: clustering by each component separately, then calculating silhouette score, calinski harabasz score, dunn score and inverse davies bouldin score for each component (feature) separately. Then scaling those scores to same magnitude, then PCA them to 1 feature. This produced weights for each component. It seems this approach produces reasonable results. I suppose better approach would be full factorial experiment (DOE), but it seems that this simple approach produces satisfactory results as well.
I am working from this article: "A novel method for predicting kidney stone type using ensemble learning". The author used a genetic algorithm for finding the optimal weight vector for voting with WEKA, but i don't know see can they did that. How can i use a genetic algorithm to find weight of voting classifier with WEKA?
This below paragraph has been extracted from the article:
In order to enhance the performance of the voting algorithm,a weighted
majority vote is used. Simple majority vote algorithm is usually an
effective way to combine different classifiers, but not all
classifiers have the same effect on the classification problem. To
optimize the results from weight majority vote classifier, we need to
find the optimal weight vector. Applying Genetic algorithms is our
solution for finding the optimal weight vector in this problem.
Assuming you have some trained classifiers and a test set, you can create a method calculateFitness(double[] weights). In this method for each Instance calculate all predictions and a merged prediction according to the weights. Use the combined predictions and the real values to calculate the total score you want to maximize/minimize.
Using the calculateFitness method you can create a custom GA to find best weights.
I have a set of labeled training data, and I am training a ML algorithm to predict the label. However, some of my data points are more important than others. Or, analogously, these points have less uncertainty than the others.
Is there a general method to include an importance-representing weight to each training point in the model? Are there instead some specific models which are capable of this while others are not?
I can imagine duplicating these points (and perhaps smearing their features slightly to avoid exact duplicates), or downsampling the less important points. Is there a more elegant way to approach this problem?
Scikit-learn allows you to pass an array of sample weights while fitting the model. Vowpal Wabbit (an online ML library) also has this option.
I am new to Text Mining. I am working on Spam filter. I did text cleaning, removed stop words. n-grams are my features. So I build a frequency matrix and build model using Naive Bayes. I have very limited set of training data, so I am facing the following problem.
When a sentence comes to me for classification and if none of its features match with the existing features in training then my frequency vector has only zeros.
When I send this vector for classification, I obviously get a useless result.
What can be ideal size of training data to expect better results?
Generally, the more data you have, the better. You will get diminishing returns at some point. It is often a good idea to see if your training set size is a problem by plotting the cross validation performance while varying the size of the training set. In scikit-learn has an example of this type of "learning curve."
Scikit-learn Learning Curve Example
You may consider bringing in outside sample posts to increase the size of your training set.
As you grow your training set, you may want to try reducing the bias of your classifier. This could be done by adding n-gram features, or switching to a logistic regression or SVM model.
When a sentence comes to me for classification and if none of its features match with the existing features in training then my frequency vector has only zeros.
You should normalize your input so that it forms some kind of rough distribution around 0. A common method is to do this tranformation:
input_signal = (feature - feature_mean) / feature_stddev
Then all zeroes would only happen if all features were exactly at the mean.
I am working on a classification problem, which has different sensors. Each sensor collect a sets of numeric values.
I think its a classification problem and want to use weka as a ML tool for this problem. But I am not sure how to use weka to deal with the input values? And which classifier will best fit for this problem( one instance of a feature is a sets of numeric value)?
For example, I have three sensors A ,B, C. Can I define 5 collected data from all sensors,as one instance? Such as, One instance of A is {1,2,3,4,5,6,7}, and one instance of B is{3,434,534,213,55,4,7). C{424,24,24,13,24,5,6}.
Thanks a lot for your time on reviewing my question.
Commonly the first classifier to try is Naive Bayes (you can find it under "Bayes" directory in Weka) because it's fast, parameter less and the classification accuracy is hard to beat whenever the training sample is small.
Random Forest (you can find it under "Tree" directory in Weka) is another pleasant classifier since it process almost any data. Just run it and see whether it gives better results. It can be just necessary to increase the number of trees from the default 10 to some higher value. Since you have 7 attributes 100 trees should be enough.
Then I would try k-NN (you can find it under "Lazy" directory in Weka and it's called "IBk") because it commonly ranks amount the best single classifiers for a wide range of datasets. The only issues with k-nn are that it scales badly for large datasets (> 1GB) and it needs to fine tune k, the number of neighbors. This value is by default set to 1 but with increasing number of training samples it's commonly better to set it up to some higher integer value in range from 2 to 60.
And finally for some datasets where both, Naive Bayes and k-nn performs poorly, it's best to use SVM (under "Functions", it's called "Lib SVM"). However, it can be hassle to set up all the parameters of the SVM to get competitive results. Hence I leave it to the end when I already know what classification accuracies to expect. This classifier may not be the most convenient if you have more than two classes to classify.