Showing posts with label: data_science. Show all posts.

I am still working on my face autoencoder in my spare time, although I have much less spare time lately. My non-variational autoencoder works great - it can very accurately reconstruct any face in my dataset of 400,000 faces, but it doesn't work at all for interpolation or anything like that. So I have also been trying to train a variational autoencoder, but it has a lot more difficulty learning.

For a face which is roughly centered and looking in the general direction of the camera it can do a somewhat decent job, but if the picture is off in any way - there is another face off to the side, there is something blocking the face, the face is at a strange angle, etc it does a pretty bad job. And since I want to try to use this for interpolation training it on these bad faces doesn't really help anything.

One of the biggest datasets I am using is this one from ETHZ. The dataset was created to train a network to predict the age of the person, and while the images are all of good quality it does include many images that have some of the issues I mentioned above, as well as pictures that are not faces at all - like drawings or cartoons. Other datasets I am using consist entirely of properly cropped faces as I described above, but this dataset is almost 200k images, so omitting it completely significantly reduces the size of my training data.

The other day I decided I needed to improve the quality of my training dataset if I ever want to get this variational autoencoder properly trained, and to do that I need to filter out the bad images from the ETHZ IMDB dataset. They had already created the dataset using face detectors, but I want to remove faces that have certain attributes:

  • Multiple faces or parts of faces in the image
  • Images with something blocking part of the face
  • Images where the faces are not generally facing forward, such as profiles

I started trying to curate them manually, but after going through 500 images of the 200k I realized that would not be feasible. It would be easy to train a neural network to classify the faces, but that would require training data, but that still means manually classifying the faces. So, what I did is I took another dataset of faces that were all good and added about 700 bad faces from the IMDB dataset for a total size of about 7000 images and made a new dataset. Then I took a pre-trained discriminator I had previously used as part of a GAN to try to generate faces and retrained it to classify the faces as good or bad. 

I ran this for about 10 epochs, until it was achieving very good accuracy, and then I used it to evaluate the IMDB dataset. Any image which it gave a less than 0.03 probability of being good I moved into the bad training dataset, and any images which it gave a 0.99 probability of being good I moved to the good training dataset. Then I continued training it and so on and so on.

This is called weak supervision or semi-supervised learning, and it works a lot better than I thought it would. After training for a few hours, the images which are moved all seem to be correctly classified, and after each iteration the size of the training dataset grows to allow the network to continue learning. Since I only move images which have very high or very low probabilities, the risk of a misclassification should be relatively low, and I expect to be able to completely sort the IMDB dataset by the end of tomorrow, maybe even sooner. What would have taken weeks or longer to do manually has been reduced to days thanks to transfer learning and weak supervision!

Labels: coding, data_science, machine_learning, pytorch, autoencoders
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CatBoost

Thursday 10 January 2019

Usually when you think of a gradient boosted decision tree you think of XGBoost or LightGBM. I'd heard of CatBoost but I'd never tried it and it didn't seem too popular. I was looking at a Kaggle competition which had a lot of categorical data and I had squeezed just about every drop of performance I could out of LGBM so I decided to give CatBoost a try. I was extremely impressed.

Out of the box, with all default parameters, CatBoost scored better than the LGBM I had spent about a week tuning. CatBoost trained significantly slower than LGBM, but it will run on a GPU and doing so makes it train just slightly slower than the LGBM. Unlike XGBoost it can handle categorical data, which is nice because in this case we have far too many categories to do one-hot encoding. I've read the documentation several times but I am still unclear as to how exactly it encodes the categorical data, but whatever it does works very well.

I am just beginning to try to tune the hyperparameters so it is unclear how much (if any) extra performance I'll be able to squeeze out of it, but I am very, very impressed with CatBoost and I highly recommend it for any datasets which contain categorical data. Thank you Yandex! 

Labels: coding, data_science, machine_learning, kaggle, catboost
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Exercise Log

Tuesday 27 November 2018

I exercise quite a lot and I have not been able to find an app to keep track of it which satisfies all of my criteria. Most fitness trackers are geared towards cardio and I also do a lot of strength training. After spending a year trying to make due with combinations of various fitness trackers and other apps I decided to just write my own, which could do everything I wanted and could show all of the reports I wanted.

I did that and after using it for a few weeks put it online at workout-log.com. It's not fancy and it is quite likely very buggy at this point, but it is open to anyone who wants to use it. 

It's written with Django and jQuery and uses ChartJS for the charts. 

Labels: python, django, data_science, machine_learning
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Early Stopping

Monday 30 July 2018

I recently began using the early stopping feature of Light GBM, which allows you to stop training when the validation score doesn't improve for a certain number of rounds. This is especially useful if you are bagging models, as you don't need to watch each one and figure out when training should stop. The way it works is you specify a number of rounds, and if the validation score doesn't improve during that number of rounds the training is stopped and the round with the best validation score is used.

When working with this I noticed that often the best validation round is a very early round, which has a very good validation score but an incredibly low training score. As an example here is the output from a model I am currently training. Normally the training F1 gets up to the high 0.90s:

Early stopping, best iteration is:
[7]	train's macroF1: 0.525992	valid's macroF1: 0.390373

Out of at least 400 rounds of training, the best performance on the validation set was on the 7th, at which time it was performing incredibly poorly on the training data. This indicates overfitting to the validation set, which is just as bad as overfitting to the training set in that the model is not likely to generalize well.

So what to do about this issue? The obvious solution would be to provide a minimum number of rounds and begin to monitor the validation score for early stopping once that number of rounds has passed, but I don't see any way to do this through the LGB API. 

I am running this code using sklearn's joblib to do parallel processing, so I have create a list of the estimators to fit and then pass that list to the parallel processing which calls a function which fits the estimator to the data and returns it. The early stopping is taken care of by LGB, so what I did is after the estimator is fit I manually get the validation results and the train performance for the best validation round. If the train performance is above a specified threshold I return the estimator as normal. If, however, the train performance is below that threshold I recursively call the function again. 

The downside to this is that it is possible to get into an infinite loop, but if the thresholds are properly tuned this should be easily avoidable. 

 

Labels: coding, data_science, machine_learning, lightgbm
1 comments

I recently started looking at a Kaggle Challenge about predicting poverty levels in Costa Rica. I used sklearn train_test_split to split the training data into train and validation sets and fit a few models. The first thing I noticed was that my submissions scored significantly lower than my validation sets: 0.36 on the submission vs. .96 on my validation data.

The data consists of information about individuals with the target as their poverty level. The features include both information relating to that individual as well as information for the household they live in. The data includes multiple individuals from the same household, and some exploratory data analysis indicated that most of the features were on a household level rather than the individual level.

This means that doing a random split ends up including data from the same household in both the train and validation datasets, which will result in the leakage that artificially raised my initial validation scores. This also means that my models were all tuned on a validation dataset which was essentially useless.

To fix this I did the split on unique household IDs, so no household would be included in both datasets. After re-tuning the models appropriately, the validation f1 scores had gone down from 0.96 to 0.65. The submissions scores went up to 0.41, which was not a huge increase, but it was much closer to the validation scores.

The moral of this story is never forget to make sure that your training and validation sets don't contain overlap or leakage, or the validation set becomes useless.

Labels: data_science, machine_learning, kaggle
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