The Economist has a short but sweet article on a new study that has found that asking the same person to make two guesses and averaging the answer is more accurate than any one guess alone, with more time between guesses improving accuracy.
The study is apparently by psychologists Hal Pashler and Ed Vul and has just been published in Psychological Science, but unfortunately the journal website is down at the moment, but I shall link to the original study when it reappears.
According to The Economist though, here’s the punchline:
The two researchers asked 428 people eight questions drawn from the “CIA World Factbook”: for example, “What percentage of the world’s airports are in the USA?” Half the participants were unexpectedly asked to make a second, different guess immediately after they completed the initial questionnaire. The other half were asked to make a second guess three weeks later.
Dr Vul and Dr Pashler found that in both circumstances the average of the two guesses was better than either guess on its own. They also noticed that the interval between the first and second guesses determined how accurate that average was. Second guesses made immediately improved accuracy by an average of 6.5%; those made after three weeks improved the accuracy by 16%.
Link to Economist article ‘The crowd within’.
Mind Hacks 
Part of the study seems flawed. A significant portion of the people who were asked for a second estimate three weeks later probably had already googled the answer.
Warm regards,
D.