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Keras oversampling

By | 05.10.2020

By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I'm trying to do a binary classification problem with Keras, using the ImageDataGenerator. However, my classes are very imbalanced, like about 8x or 9x more in one class than the other, causing the model to get stuck predicting the same output class for every example.

For now, I've just created multiple copies of each image in my smaller class, but I'd like to have a bit more flexibility. With current version of Keras - it's not possible to balance your dataset using only Keras built-in methods. But you could do a different trick - by writting your own generator which would make the balancing inside the python :.

For most of the applications the size of the batch doesn't need to be the same - but there are some weird use cases like e. Learn more. Asked 3 years, 2 months ago. Active 1 year, 8 months ago. Viewed 5k times. George George 1, 1 1 gold badge 9 9 silver badges 22 22 bronze badges. Active Oldest Votes. Can someone create a fully working script based on this, to elaborate? Pasha Dembo Pasha Dembo 2 2 silver badges 1 1 bronze badge.

How can you guarantee np. Michael Michael 10 10 bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name.This tutorial demonstrates how to classify a highly imbalanced dataset in which the number of examples in one class greatly outnumbers the examples in another. The aim is to detect a mere fraudulent transactions fromtransactions in total.

You will use Keras to define the model and class weights to help the model learn from the imbalanced data. Pandas is a Python library with many helpful utilities for loading and working with structured data and can be used to download CSVs into a dataframe.

The raw data has a few issues. First the Time and Amount columns are too variable to use directly. Drop the Time column since it's not clear what it means and take the log of the Amount column to reduce its range.

Split the dataset into train, validation, and test sets. The validation set is used during the model fitting to evaluate the loss and any metrics, however the model is not fit with this data.

The test set is completely unused during the training phase and is only used at the end to evaluate how well the model generalizes to new data. This is especially important with imbalanced datasets where overfitting is a significant concern from the lack of training data.

Normalize the input features using the sklearn StandardScaler. This will set the mean to 0 and standard deviation to 1. Next compare the distributions of the positive and negative examples over a few features. Good questions to ask yourself at this point are:.

keras oversampling

Define a function that creates a simple neural network with a densly connected hidden layer, a dropout layer to reduce overfitting, and an output sigmoid layer that returns the probability of a transaction being fraudulent:.

Notice that there are a few metrics defined above that can be computed by the model that will be helpful when evaluating the performance.

Now create and train your model using the function that was defined earlier. Notice that the model is fit using a larger than default batch size ofthis is important to ensure that each batch has a decent chance of containing a few positive samples.

If the batch size was too small, they would likely have no fraudulent transactions to learn from. These are initial guesses are not great. You know the dataset is imbalanced. This can help with initial convergence. With the default bias initialization the loss should be about math. This way the model doesn't need to spend the first few epochs just learning that positive examples are unlikely.

This also makes it easier to read plots of the loss during training. To make the various training runs more comparable, keep this initial model's weights in a checkpoint file, and load them into each model before training. Train the model for 20 epochs, with and without this careful initialization, and compare the losses:.

The above figure makes it clear: In terms of validation loss, on this problem, this careful initialization gives a clear advantage. In this section, you will produce plots of your model's accuracy and loss on the training and validation set. These are useful to check for overfitting, which you can learn more about in this tutorial.

Additionally, you can produce these plots for any of the metrics you created above. False negatives are included as an example.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Data Science Stack Exchange is a question and answer site for Data science professionals, Machine Learning specialists, and those interested in learning more about the field.

It only takes a minute to sign up. Would somebody so kind to provide one?

keras oversampling

By the way, in this case the appropriate praxis is simply to weight up the minority class proportionally to its underrepresentation? If you are talking about the regular case, where your network produces only one output, then your assumption is correct. In order to force your algorithm to treat every instance of class 1 as 50 instances of class 0 you have to:.

EDIT: "treat every instance of class 1 as 50 instances of class 0 " means that in your loss function you assign higher value to these instances. Adjust accordingly when copying code from the comments. That means that you should pass a 1D array with the same number of elements as your training samples indicating the weight for each of those samples.

This means you should pass a weight for each class that you are trying to classify. If you need more than class weighting where you want different costs for false positives and false negatives. With the new keras version now you can just override the respective loss function as given below.

Classification on imbalanced data

Note that weights is a square matrix. I found the following example of coding up class weights in the loss function using the minist dataset.

This works with a generator or standard. Your largest class will have a weight of 1 while the others will have values greater than 1 depending on how infrequent they are relative to the largest class. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered.

How to set class weights for imbalanced classes in Keras? Ask Question. Asked 3 years, 8 months ago. Active 6 days ago. Viewed k times. Hendrik Hendrik 5, 13 13 gold badges 33 33 silver badges 49 49 bronze badges. Active Oldest Votes.Last Updated on April 7, Imbalanced classification involves developing predictive models on classification datasets that have a severe class imbalance.

The challenge of working with imbalanced datasets is that most machine learning techniques will ignore, and in turn have poor performance on, the minority class, although typically it is performance on the minority class that is most important. One approach to addressing imbalanced datasets is to oversample the minority class. Instead, new examples can be synthesized from the existing examples.

Discover SMOTE, one-class classification, cost-sensitive learning, threshold moving, and much more in my new bookwith 30 step-by-step tutorials and full Python source code.

A problem with imbalanced classification is that there are too few examples of the minority class for a model to effectively learn the decision boundary. One way to solve this problem is to oversample the examples in the minority class. This can be achieved by simply duplicating examples from the minority class in the training dataset prior to fitting a model. This can balance the class distribution but does not provide any additional information to the model.

An improvement on duplicating examples from the minority class is to synthesize new examples from the minority class.

eXtreme Gradient Boosting XGBoost Algorithm with R - Example in Easy Steps with One-Hot Encoding

This is a type of data augmentation for tabular data and can be very effective. This technique was described by Nitesh Chawlaet al. SMOTE works by selecting examples that are close in the feature space, drawing a line between the examples in the feature space and drawing a new sample at a point along that line.

Specifically, a random example from the minority class is first chosen. A randomly selected neighbor is chosen and a synthetic example is created at a randomly selected point between the two examples in feature space.

The synthetic instance is then created by choosing one of the k nearest neighbors b at random and connecting a and b to form a line segment in the feature space. The synthetic instances are generated as a convex combination of the two chosen instances a and b. This procedure can be used to create as many synthetic examples for the minority class as are required.

As described in the paper, it suggests first using random undersampling to trim the number of examples in the majority class, then use SMOTE to oversample the minority class to balance the class distribution.

The approach is effective because new synthetic examples from the minority class are created that are plausible, that is, are relatively close in feature space to existing examples from the minority class.

Our method of synthetic over-sampling works to cause the classifier to build larger decision regions that contain nearby minority class points. A general downside of the approach is that synthetic examples are created without considering the majority class, possibly resulting in ambiguous examples if there is a strong overlap for the classes.

In these examples, we will use the implementations provided by the imbalanced-learn Python librarywhich can be installed via pip as follows:. You can confirm that the installation was successful by printing the version of the installed library:. In this section, we will develop an intuition for the SMOTE by applying it to an imbalanced binary classification problem. We can use the Counter object to summarize the number of examples in each class to confirm the dataset was created correctly.

Finally, we can create a scatter plot of the dataset and color the examples for each class a different color to clearly see the spatial nature of the class imbalance. Tying this all together, the complete example of generating and plotting a synthetic binary classification problem is listed below.

Running the example first summarizes the class distribution, confirms the ratio, in this case with about 9, examples in the majority class and in the minority class. A scatter plot of the dataset is created showing the large mass of points that belong to the minority class blue and a small number of points spread out for the minority class orange.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. The dark mode beta is finally here.

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keras oversampling

Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. The tutorial here demonstrates how a small but balanced dataset can be augmented using the ImageDataGenerator. Is there an easy way to use this generator to augment a heavily unbalanced dataset, such that the resulting, generated dataset is balanced?

This would not be a standard approach to deal with unbalanced data. Nor do I think it would be really justified - you would be significantly changing the distributions of your classes, where the smaller class is now much less variable.

The larger class would have rich variation, the smaller would be many similar images with small affine transforms. They would live on a much smaller region in image space than the majority class. The first two options are really kind of hacks, which may harm your ability to cope with real world imbalanced data.

Neither really solves the problem of low variability, which is inherent in having too little data. If application to a real world dataset after model training isn't a concern and you just want good results on the data you have, then these options are fine and much easier than making generators for a single class.

If you truly want to generate a variety of augmented images for one class over another, it would probably be easiest to do it in pre-processing. Take the images of the minority class and generate some augmented versions, and just call it all part of your data. Like I say, this is all pretty hacky. Learn more. Asked 3 years, 3 months ago. Active 1 month ago. Viewed 10k times. The keras ImageDataGenerator can be used to " Generate batches of tensor image data with real-time data augmentation " The tutorial here demonstrates how a small but balanced dataset can be augmented using the ImageDataGenerator.

The Right Way to Oversample in Predictive Modeling

Anshuman Kumar 1 1 gold badge 3 3 silver badges 15 15 bronze badges. Active Oldest Votes. Deep learning can cope with this, it just needs lots more data the solution to everything, really. Utkarsh Sinha 3, 4 4 gold badges 25 25 silver badges 42 42 bronze badges. Thanks a lot for sharing your insight.

I will look into that google paper. Sign up or log in Sign up using Google.Imbalanced datasets spring up everywhere. Amazon wants to classify fake reviews, banks want to predict fraudulent credit card charges, and, as of this November, Facebook researchers are probably wondering if they can predict which news articles are fake. In each of these cases, only a small fraction of observations are actually positives.

Recently, oversampling the minority class observations has become a common approach to improve the quality of predictive modeling. By oversampling, models are sometimes better able to learn patterns that differentiate classes.

Since one of the primary goals of model validation is to estimate how it will perform on unseen data, oversampling correctly is critical. I know this dataset should be imbalanced most loans are paid offbut how imbalanced is it? With the data prepared, I can create a training dataset and a test dataset. After upsampling to a class ratio of 1.

But is this actually representative of how the model will perform? To see how this works, think about the case of simple oversampling where I just duplicate observations. If I upsample a dataset before splitting it into a train and validation set, I could end up with the same observation in both datasets.

As a result, a complex enough model will be able to perfectly predict the value for those observations when predicting on the validation set, inflating the accuracy and recall. However, because the SMOTE algorithm uses the nearest neighbors of observations to create synthetic data, it still bleeds information. If the nearest neighbors of minority class observations in the training set end up in the validation set, their information is partially captured by the synthetic data in the training set.

As a result, the model will be better able to predict validation set values than completely new data. By oversampling only on the training data, none of the information in the validation data is being used to create synthetic observations. So these results should be generalizable. The validation results closely match the unseen test data results, which is exactly what I would want to see after putting a model into production.

Oversampling is a well-known way to potentially improve models trained on imbalanced data. Random forests are great because the model architecture reduces overfitting see Brieman for a proofbut poor sampling practices can still lead to false conclusions about the quality of a model.

The main point of model validation is to estimate how the model will generalize to new data. Faster Web Scraping in Python with Multithreading. Software product development lessons fromblog readers. Or, why point estimates only get you so far. Validation Results 0. Leave a Comment.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Already on GitHub? Sign in to your account. I have an image dataset with an unbalanced class distribution: certain common classes have up to 10x as many samples as certain uncommon classes.

I would like to rebalance what images my classifier is exposed to by using some combination of oversampling and undersampling methods from imbalanced-learn. While this might not be something worth breaking backwards compatibility over in terms of default behavior, it'd be great if I had a switch to turn this deduplication behavior off.

As far as I can tell this would be the easiest way to incorporate data resampling technique into my model training workflow; the workaround would be to manually copy the images on disk, which is super dissatisfying.

If that sounds like a good idea, maybe I can whip something up. Also i recommend you not to over or under sample but to use class weights to influence the loss function. Skip to content. Dismiss Join GitHub today GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together. Sign up. New issue.

Random Oversampling and Undersampling for Imbalanced Classification

Jump to bottom. Labels image. Copy link Quote reply. ResidentMario added the image label Mar 8, This comment has been minimized. Sign in to view. It's already a parameter!

SMOTE for Imbalanced Classification with Python

Now I feel very foolish indeed. Thank you. Dref added a commit that referenced this issue Mar 13,

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