New answers tagged class-imbalance
2
My question is: Does it matter if the minority class is set as the positive or negative label in relation to performance of training a model or affecting a loss function such as cross entropy?
No it doesn't.
However in binary classification it's customary to call "positive" the main class of interest, so be careful to be clear about which one is ...
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Some Python Sklearn models have this option : class_weight="balanced".
By that, you specify to your algorithm your data are unbalanced, and it makes the changes by itself. You can try this on few models, I had a better result with this option than by using the Downsampling Majority Class technique in a same problem
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Referring to a previous answer and a blog post (which I'm aware is not that relevant since the data is more balanced than yours), I think that your first approach should be without handling imbalance, and if you're happy with the results, no need to work towards balanced solutions.
As in many ML topics, the best way is to try, I recommend you to adapt the ...
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Referring to a previous question, there is no reason to tackle imbalance unless your model is not learning properly with the imbalanced dataset. Besides, 1:7 is not that big of an imbalance.
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Referring to an answer to a similar question, you don't have any reason to handle unbalance from the beginning. An imbalance of 95:5 isn't that big, I'd start with the regular training and if that doesn't work try more sophisticated things.
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The point of setting class weights is to manipulate the loss function to put more focus on the minor label. In fact, each of the data point passed to your learning algorithm will contribute information to help your loss function. By making the weight of a minor instance bigger, you say to your loss function that it should put more focus on that particular (...
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I am not sure about the model you are using but I might explain what the procedure is for ML in general. You have three "vanilla" solutions for coping with unbalanced supervised dataset.
Reweighing class label so that there is the same number (calculated as sum of weights for given label) of samples per label. For example if a label with maximum ...
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With such a heavy imbalance and two classes (it seems) you could treat this as more of an outlier detection problem. You should read up on models and algorithms in that direction!
If you go forward with a traditional classification you need to balance the data set, consider methods such as SMOTE.
Depending on the size of your data I would generally recommend ...
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Supporting the second answer by Donald, I would also try to look at the predicted probabilities (via the predict_proba attribute in scikit learn classifiers) and customize your predictions by selecting the threshold (which you can check also in the corresponding ROC curve) which gives you the most robust (i.e. highest) predicted probabilities distributions, ...
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Not clear if you are saying the cost of a FN or a FP is higher, you only mention FN in your statement. Think that you mean a FN is more costly and that a positive means a 1.
In general, if an incorrect prediction for the minority case is more costly (FN), you should sample that minority case higher or the majority case lower so the ratio is closer to 1:1. ...
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Ideally, the threshold should be selected on your training set. Your holdout set is just there to double confirm that whatever has worked on your training set will generalize to images outside of the training set.
This is the reason why hyperparameters tuning like GridSearch and RandomizedSearch in python has a cv parameter to cross-validate between ...
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Class imbalance is a frequent problem in machine learning and techniques to balance the data usualy are of two flavors: undersampling the majority, oversampling the minority or both.
One can always partition the data according to some variables and separately oversample each partition so as to maintain some measure (eg given data distribution). In the same ...
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