Available on Xbox LIVE in time for the holidays, "Karaoke" for Xbox 360 will have a selection of free sample songs that will change frequently.

ENGADGET: Karaoke for Xbox Live brings pay to play sing-alongs to your holiday parties

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

Most stalls tend to specialise in one particular dish, but between them they offer an opportunity to sample a good selection of Morocco's traditional food.

BBC: Djemaa el Fna, the beating heart of Marrakesh