Statistically Efficient Ways to Quantify Added Predictive Value of New Measurements

If I understand your process correctly, I think you will need to seriously evaluate if you have enough observations to do the validation by holding out data. Much of the recommendations of @f2harrell are based upon important papers by Julian Faraway:

Faraway, J. J. (1992). On the cost of data analysis. Journal of Computational and Graphical Statistics, 1(3), 213-229. (PDF)

A complete discussion on the use of split data methods must also include:

Faraway, J. J. (2016). Does data splitting improve prediction?. Statistics and computing, 26, 49-60. (PDF)

At least for medical applications, a strong argument can be made that split data approaches lose too much information relative to model specification and bootstrap validation.

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