I have some data (eg. Titanic) and I want to use logistic regression to predict probability of survive. I have problem to understand the difference between model development and model validation. Firstly we have to choose which variables use in our model and to do it we need to have tool which shows how good is our model. What is this tool in logistic regression? If I have for example 10 independent variables, should I delete some insignificant variables or it is okay to keep it in the model? How to choose which varables delete? We also have to choose in some way "threshold" to decide if someone survive or not. We do it during model development or during model validation? When model development ends and starts model validation?