# What are development tests used for?

This is a theoretical question. I am newbie to artificial intelligence and machine learning, and the more I read the more I like this. So far, I have been reading about evaluation of language models (I am focused on ASR), but I still don't get the concept of development test. The clearest explanation I have come across is the following

"Sometimes we use a particular test set so often that we implicitly tune to its
characteristics. We then need a fresh test set that is truly unseen. In such cases,
we call the initial test set the development test set or, devset"


Nevetheless I have not found sense as for why an additional test has to be used. In other words, why aren't training and test sets enough?

Thanks in advance!

• Maybe same reasoning as for writing a legacy application from scrath again when time flies over too many years. – mico Mar 13 '18 at 12:48

## 1 Answer

In machine learning you normaly split your data into 3 parts(80-10-10%). First part is for the training of your ML-model. The second part (10%) is the development set (or validation set). This is used as measuring your performance with various hyper parameters (e.g. in neural networks: layer size). After you found your best hyper parameters, you learn the model again on the test set and then test it on your test data (10%) which the model has never seen before. Your result on the test data is now a good indicator how your model prediction quality is in the real worl (because it was never optimized for this test data).

• Thank you for your response. It was pretty clear and succinct! – little_mice Mar 13 '18 at 17:12