Vintage players tend to be good, because of a selection effect: If you use a cane and still have a job in the major leagues, it's usually because you can play.

WSJ: New York Yankees: Getting Old Is Nothing to Fear��Tim Marchman

Mr Heckman confronted the sample-selection problem: in terms of the examples just given, for instance, the effect on the results if the workers who answered the questionnaire differed in important ways from those who did not.

ECONOMIST: Economics focus

But both the selection bias and regular-phone-use definition might be expected to mask any adverse effect of phones.

ECONOMIST: The Interphone study: Mobile madness | The