[BostonCard]

I should hurry up and get this thing before my 45th birthday in November!

BC

All of these studies that split the data into 5 age classes should be treating age as a 5th-degree polynomial instead.  Just as many degrees of freedom, and I'll bet the predicted mortality rates, etc. would fit the data better.

(Prediction: no one will do that)

Obviously, I am making fun of reducing a continuous predictor into a bunch of categories.  Your point is well taken, but, more importantly, you don't have to preserve the degrees of freedom; a quick perusal of the data would tell you that a simple quadratic is probably a better descriptor of the data, and it has saved you three DF's.

And/but, as always, it depends on what you want to do with the results.  An nth degree polynomial (where n > 1) makes it hard to explain what is seen, since the terms higher than the linear term are hard to describe, unless they are all positive.  More importantly, while as a 44-year-old, it is pretty easy to look at my risk category and infer that I would likely be at the upper end of the range for the 18-44 year old category, while more precise, it would be much harder to calculate my risk with even a quadratic polynomial, absent a calculator.  So there is a tradeoff between precision and ease of understanding.

BC

[BostonCard]

I should hurry up and get this thing before my 45th birthday in November!

BC

All of these studies that split the data into 5 age classes should be treating age as a 5th-degree polynomial instead.  Just as many degrees of freedom, and I'll bet the predicted mortality rates, etc. would fit the data better.

(Prediction: no one will do that)

Obviously, I am making fun of reducing a continuous predictor into a bunch of categories.  Your point is well taken, but, more importantly, you don't have to preserve the degrees of freedom; a quick perusal of the data would tell you that a simple quadratic is probably a better descriptor of the data, and it has saved you three DF's.

And/but, as always, it depends on what you want to do with the results.  An nth degree polynomial (where n > 1) makes it hard to explain what is seen, since the terms higher than the linear term are hard to describe, unless they are all positive.  More importantly, while as a 44-year-old, it is pretty easy to look at my risk category and infer that I would likely be at the upper end of the range for the 18-44 year old category, while more precise, it would be much harder to calculate my risk with even a quadratic polynomial, absent a calculator.  So there is a tradeoff between precision and ease of understanding.

BC