# When does model validation with a global test set / global hold out does not suffer from variance problem?

My question refers to model validation.

It is often suggested to split data into three data sets (training, validation and global test set). Training and validation data can be used for model selection and hyperparameter tuning to identify the best model, the performance of which is subsequently evaluated on the global test set.

However, if you have few data (small $n$), evaluating model performance usually suffers from a variance problem (i.e. the performance depends heavily on the way the test set is choosen). My understanding is that cross-validation is one way of addressing this problem. Since cross-validation can also be used for tuning hyperparameters of models, one may e.g. use an inner-cross validation for hyperparameter tuning and an outer cross valdiation for model validation to address the variance problem.

Essentially my problem goes back to the question under what circumstances the use of a single global test set gives reasonable indication of model performance at all. In the second reference it was quoted that $n$ should at least be 20,000. However, in order to reduce both bias and variance, it is required to trade-off the size of training and test set. So should validation not always involve some kind of resampling methods? Is there any literature on this problem?

Here is a number of related posts on the topic

Cross validation with test data set

Do I need a global test set when using k-fold cross validation?

if you have few data where n≤p or n is not sufficiently greater than p, evaluating model performance usually suffers from a variance problem (i.e. the performance depends heavily on the way the test set is choosen).

I think there is a misunderstanding here: measuring the performance of a given model depends on the absolute number of test cases (and on how representative the test set is wrt. the application), but not at the model complexity.

Model complexity enters this question only in the way that too complex or not complex enough models have low performance - and that the confidence interval for the performance estimate may depend on both the actual (true) performance of the model and the number of test cases.

For details on how to use this to calculate necessary test sample sizes for proportion-type figures of merit (e.g. sensitivity, specificity, accuracy) for different application scenarios, please have a look at our paper:
Beleites, C. and Neugebauer, U. and Bocklitz, T. and Krafft, C. and Popp, J.: Sample size planning for classification models. Anal Chim Acta, 2013, 760, 25-33. DOI: 10.1016/j.aca.2012.11.007
accepted manuscript on arXiv: 1211.1323

One interesting point here is that the confidence intervals are widest for p = 50 %, so if you determine your sample size with that, it will be sufficient with whatever model performance you observe when you have your data. (You could also plug in some estimate of what performance you expect to achieve.)

### Is it possible (and how) to determine beforehand whether CV or hold out should be used?

There are a few points that help with the decision:

• First of all, if the hold-out set is produced from available data with a random split similar to the splitting for CV, that hold out is almost always worse than some kind of resampling validation: you have all the drawbacks (dependence issues) you have with resampling, plus lower test sample size - and don't use any of the possible advantages of validating with a truly independent test set.

• some reasons for going for an independent test set (as opposed to simple splitting for hold-out)

• There's IMHO a crucial difference depending on what the purpose of your testing is, i.e. in which way you want to generalize your findings.

• for measuring the predictive abilities of the model produced from this data set, the dependence between the surrogate models in resampling validation is wanted, so go for cross validation.
• if you want to generalize what performance the given algorithm can achieve with similar data sets (but not the one at hand), then the dependence between the surrogate models makes your estimate overoptimistic, and you may be better or with truly independent test data.
• If your total available sample size is too small, so the confidence interval for hold-out results are so wide that hold-out results are useless (I have to tell people that if they reserve 3 test patients only, the confidence interval will range all the way from worse-than-guessing to perfect), there's no choice but resampling.
(If the sample size is so small that neither resampling validation results are useful => get more samples)

• First of all, that is an execellent piece of work and I agree that the question was a little bit ill-phrased. Regarding your reference: I understand the idea of testing test sample sizes based on performance confidence intervals. However, this implies that you cannot determine when to use CV vs. a global hold out in advance, correct? So you would need to do that at least once for each specific application scenario to then infer on the required test sample size, correct? – BjoSch Mar 23 '17 at 8:48
• @BjoSch: thanks for the flowers :-). I added some points that help with the decision, and yes it is possible to determine this beforehand. – cbeleites Mar 24 '17 at 11:06