In other words, some machine-based tutoring is approaching the effect size of real human tutoring and there is less variation than one might expect as the grain size of tutoring becomes finer.

FORBES: How Machine-Based Tutoring Could Disrupt Human Tutors

If the same techniques were applied across the world, the size of the oceanic carbon sink, and its variation year by year, would be known considerably better than the size of the land sink.

ECONOMIST: Climate change

In a 2009 paper Mr Glaeser and Matthew Resseger of Harvard University find that in highly skilled areas, city size explains 45% of the variation in worker productivity (it has almost no effect in underskilled cities).

ECONOMIST: Free exchange

Ideally, currency zones should be compact and homogenous enough to show little regional variation in business cycles otherwise a one-size-fits-all monetary policy will leave some regions lingering in recession, while others grow so fast they overheat.

ECONOMIST: Can this union be saved?