matlab - Concept of validate for neural network

Question

I have a problem with concept of Validation for NN. suppose I have 100 set of input variables (for example 8 input, X1,...,X8) and want to predict one Target(Y). now I have two ways to use NN: 1- use 70 set of data for training NN and then use trained NN to predict other 30 sets of Target for validation and then plot output VS Target for this 30 sets as validation plot. 2- use 100 sets of data for training NN and then divide all outputs to two part (70% and 30%). plot 70% of outputs VS corresponding Targets as Training plot. then plot other 30% outputs VS their corresponding Targets as validation plot

Which one is correct??

Also, what the difference between checking NN with new data set and validation data set??

Thanks

Answer 1

You cannot use data for validation, if it has been already used for the training, because the trained NN will already "know" your validation examples. The result of such validation will be very biased. I would for sure use the first way.