Training/Dev/Test Set what is training set / dev set / test set In traditional methodology/ when we have small size data we can take 60-20-20 ratio to get training set-validation set/dev set -test set. Now, when we have big data it is fine that the dev set or test set to be less than 10 or 20 percent of your data. Or even 98-1-1 ratio is also fine. One rule of thumb is : Test set and Dev set should come from same distribution. Bias and Variance Bias means the high error rate in training. I may be due to underfitting. For this we can change neural network architecture like network size and number of iterations. Varaince means error rate in Dev set . This may be due to Over fitting of the data . This can be avoided by increasing number of data and regularization. Bias - Variance trade off means balancing both without the increase in other. Regularization is used to reduce the variance . It may hurt bias and bias may increase a little but not much if we have a bigger network...