 # Statistically Efficient Ways to Quantify Added Predictive Value of New Measurements

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Hi Dr. Harrell, I greatly enjoyed the post. I was hoping to apply one of the metrics to a project I am working on examining whether a measure of cognition adds predictive value to an established mortality prognostic model in older adults. In this project, I calculated the fraction of new information provided to be 0.32 (1 - (1.02/1.51), with histograms shown below.

I was curious how a medical audience might interpret this value - is it just a judgement call if the fraction is high enough? Are there other metrics you would suggest? Any thoughts would be much appreciated.

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Great question, Ashwin. If the variable being added was pre-specified, and the base model includes all the usual variables, this is quite a large amount of additional information. I think that a medical audience should be impressed. You might supplement this with a scatterplot of y=predicted value from the combined model vs. x=predicted value from old variables alone. You can compute from this the mean absolute change of predictions, and possible also show a high-resolution histogram of those absolute changes. So this might be a 4-panel plot.

One technical detail. For a binary or ordinal outcome, it is best to compute the variances on the probability scale, and to possibly use that scale in your plots.

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Hi Dr. Harrell,

Similar praises as Ashwin for the post. Also, thanks to him for asking a great question.

I am imagining if he were to try to evaluate another measure of cognition (Prognostic Index + Condition_Measure_B). He would then try to compare that model with the model above . . . in other words, non-nested comparison. Are there simple gold-standards for trying to evaluate if Cognition Measure A adds more information for predictions than Cognition Measure B?

I was thinking some possibilities might be . . .

I. There could be clear-cut cases where the LR Test is statistically significant (I know, I know) for the base model vs. Prognostic Index + Measure A but not for the base model vs. Prognostic Index + Measure B
2. Visually comparing their histograms, noting differences in validation indices, etc.
3. Testing in a validation sample

If this IS possible to do, a follow-up blog post for non-nested models would be beyond amazing. Anyway, many thanks in advance.

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Hi Kip - Someday I hope to do justice to that question by adding to the blog post. I do get into that in the Maximum Likelihood chapter of my RMS book where I show some decompositions of the likelihood ratio \chi^2 statistic for a grand model, and also discuss the following: Create two models that are nested within the super (grand) model that contains both measures A and B. You get a clear answer with a likelihood ratio \chi^2 test if A adds to B but B does not add to A. If they both add to each other then you have evidence of needing both measurements.

I’d still use some of the indexes discussed in the blog post for informal comparisons of non-nested models.

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It might be a silly question… but what to do if I have three candidate markers that I want to investigate?

Let’s call the set of common covariates X, and the three markers A, B and C.

When reading the blog post, my original idea was that we will have three adequacy indices: LR_{X}/LR_{XA}, LR_{X}/LR_{XB} and LR_{X}/LR_{XC}.

However, the book (and especially the paper cited there, Califf et al 1985) suggests otherwise: namely LR_{XA}/LR_{XABC}, LR_{XB}/LR_{XABC} and LR_{XC}/LR_{XABC} if my understanding is correct.

So, I might be totally overlooking something here, but what is the sound approach…?

It all depends on whether you are interested in measuring individual added value of a biomarker, or are interested in having the biomarkers compete not only with X but with each other.

has anyone interacted with climate scientists to see how they go about it? i saw Andy Grieve tweet this article Climate Science Needs Professional Statisticians, so i glanced quickly at their literature: Model Variable Augmentation (MVA) for Diagnostic Assessment of Sensitivity Analysis Results … i want to look into this because it wouldn’t surprise me if medical statisticians and climostatisticians are not swapping ideas, even within medicine there sometimes seems to be limited migration of ideas from one disease area to another

Thanks for the quick answer! Well the question is: “what is the best predictor?”. You might say that in clinical practice you’ll only use one predictor (that’s specifically why you’re interested to know what is the best), so I tend to accept the first interpretation, but in this sense they also compete with each other.

I don’t see the “only use one predictor” as you always need to use background variables such as age. Note that if you are trying to select or rank predictors you should use the bootstrap to get confidence intervals on the ranks as exemplified in my RMS course notes or in the BBR chapter on challenges of high-dimensional data.

Sorry, that was just a typo. I meant only use one additional predictor (i.e., in addition to X, the predictors used in every model).