Justin Wolfers writes:
Freakonomics » How to Spot Advocacy Science: John Taylor Edition: Be wary of economists wielding short samples.
And Justin comments:
Sometimes you see the perfect piece of evidence. The scatter plot that is just so. The data line up perfectly. And then you realize, perhaps they’re just too perfect. What you are seeing is advocacy, dressed up as science. Here’s an example, provided by John Taylor (via Greg Mankiw).... What conclusions should we draw about this relationship? And now why do you think Taylor began his sample in 1990? Actually, we should use all the available data. The chart below goes back to 1948, when these series—in their current form—began....
Here’s Mankiw’s assessment of Taylor’s claim:
There’s no doubt that the strength of the correlation is impressive.
But when you look beyond the cherry-picked sample, the correlation is a decidedly unimpressive -0.14.
