I am new in Statistics and Data Science. I would like to use "academic" data for training and testing for overfitting. However, I would like to get the classifier accuracy from "real-world" data and do hyperparameter optimization based on the score obtained with this data (instead of the usual grid search with cross validation). Would it be correct?


It's not really clear what you are asking, but:

  • any data you use for training your data, including hyperparameter selection, should not be used for testing
  • any data you use to train models that you use in an academic paper, from whatever sources, should be cited; this includes data used "only" for hyperparameter selection
  • use of additional data sources could be considered a type of augmentation, I suppose, but in any case whether you consider it an additional dataset, or augmentation, if you are using it in an academic paper, you need to state/cite these sources/augmentations. This lets people reproduce your results, and understand clearly what you did

If your goal is to train a model for eg an Android app, that you are writing yourself, and selling yourself, then citations would be less critical, unless you are using someone else's licensed/copyrighted data, which might require attribution/citation/license payments/etc. In that case, what you care about is generalization, runtime prediction performance. You'll want to keep a set of test data separate from whatever you used to evaluate hyperparameters etc, in order to evaluate your model. You can only use test data once in theory, or a few times, in practice, before it becomes "worn out", since you'll basically start to overfit your hyperparameters, and/or model selection, against your test set.

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