Brad DeLong should stop insulting Megan McArdle about her Math Skills

http://delong.typepad.com/sdj/2010/09/did-megan-mcardle-really-graduate-from-a-business-school.html

 

Brad DeLong really misses the point here.

 

When you're looking at PERCENTAGE OF GDP, you're looking at A sub t over GDP sub t.

 

When you look at the present value of these, however, and you're using different discount rates because of different growth rates, you end up with distorted percentages. This is very simple math.