Political poll with sample size of 47 makes headlines
David Farrar of Kiwi Blog criticises a story in the Herald which says:
John Banks has some support in the wealthy suburb of Remuera, but is less popular on the liberal fringes of the Epsom electorate, according to a Herald street survey.
A poll of 47 Epsom voters yesterday found the National candidate ahead of Act’s Mr Banks by 22 votes to 20.
Farrar correctly points out that the poll is in no way random (i.e. is not scientific), and goes on to say:
But even if you overlook the fact it is a street poll, the sample size is ridiculously low. The margin of error is 14.7%! I generally regard 300 as the minimum acceptable for an electorate poll. That gives a 5.8% margin of error. A sample of 47 is close to useless.
Rachel Cunliffe is the co-director of CensusAtSchool and currently consults for the Department of Statistics. Her interests include statistical literacy, social media and blogging. See all posts by Rachel Cunliffe »
I intuit that at n = 47 we have already reached the point where the sample selection bias dominates over random error. I can think of no reason to add to the sample by the same method!
13 years ago
Can the herald please employ somine who understands stats so we stop getting presented all this rubbish!
13 years ago