The MNIST database (Mixed National Institute of Standards and Technology database) is a large database of handwritten digits that is commonly used for training various image processing systems. The database is also widely used for training and testing in the field of machine learning. It was created by "re-mixing" the samples from NIST's original datasets. The creators felt that since NIST's training dataset was taken from American Census Bureau employees, while the testing dataset was taken from American high school students, it was not well-suited for machine learning experiments. Furthermore, the black and white images from NIST were normalized to fit into a 20x20 pixel bounding box and anti-aliased, which introduced grayscale levels.
The MNIST database contains 60,000 training images and 10,000 testing images. Half of the training set and half of the test set were taken from NIST's training dataset, while the other half of the training set and the other half of the test set were taken from NIST's testing dataset. There have been a number of scientific papers on attempts to achieve the lowest error rate; one paper, using a hierarchical system of convolutional neural networks, manages to get an error rate on the MNIST database of 0.23 percent. The original creators of the database keep a list of some of the methods tested on it. In their original paper, they use a support vector machine to get an error rate of 0.8 percent.
The set of images in the MNIST database is a combination of two of NIST's databases: Special Database 1 and Special Database 3. Special Database 1 and Special Database 3 consist of digits written by high school students and employees of the United States Census Bureau, respectively.
Some researchers have achieved "near-human performance" on the MNIST database, using a committee of neural networks; in the same paper, the authors achieve performance double that of humans on other recognition tasks. The highest error rate listed on the original website of the database is 12 percent, which is achieved using a simple linear classifier with no preprocessing.
In 2004, A best-case error rate of 0.42 percent was achieved on the database by researchers using a new classifier called the LIRA, which is a neural classifier with three neuron layers based on Rosenblatt's perceptron principles.
Some researchers have tested artificial intelligence systems using the database put under random distortions. The systems in these cases are usually neural networks and the distortions used tend to be either affine distortions or elastic distortions. Sometimes, these systems can be very successful; one such system achieved an error rate on the database of 0.39 percent.
In 2011, an error rate of 0.27 percent, improving on the previous best result, was reported by researchers using a similar system of neural networks. In 2013, an approach based on regularization of neural networks using DropConnect has been claimed to achieve a 0.21 percent error rate.
This is a table of some of the machine learning methods used on the database and their error rates, by type of classifier:
|Type||Classifier||Distortion||Preprocessing||Error rate (%)|
|Linear classifier||Pairwise linear classifier||None||Deskewing||7.6|
|K-Nearest Neighbors||K-NN with non-linear deformation (P2DHMDM)||None||Shiftable edges||0.52|
|Boosted Stumps||Product of stumps on Haar features||None||Haar features||0.87|
|Non-Linear Classifier||40 PCA + quadratic classifier||None||None||3.3|
|Support vector machine||Virtual SVM, deg-9 poly, 2-pixel jittered||None||Deskewing||0.56|
|Neural network||2-layer 784-800-10||None||None||1.6|
|Neural network||2-layer 784-800-10||elastic distortions||None||0.7|
|Deep neural network||6-layer 784-2500-2000-1500-1000-500-10||elastic distortions||None||0.35|
|Convolutional neural network||Committee of 35 conv. net, 1-20-P-40-P-150-10||elastic distortions||Width normalizations||0.23|
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