Just read the blurb and there are two problems .. one is comparing retrospective % with prospective (see below) and the other is inferring causation. But it's probably the reporters who blotched this; they probably cherry picked % from a lengthy interview and the result makes no sense.
I sometimes spent class time discussing pro- versus retrospective studies, and also time doing 2 by 2 boxes with different base rates. If you take their "69% of Candy eaters were violent" and "42% of non-violent children ate candy" and construct a table, and arbitrarily assign "100" to represent the # of candy eaters, certain features of the box are fixed, namely Violent Nonviolent Candy 69 31 100 No Candy __________________42___ 73 You can make things come out differently depending on the base rate of candy eating. If only 33%, it follows that: Violent Nonviolent Candy 69 31 100 No Candy _____158__________42__ 200 227 73 300 It's still true that 69% of candy eaters are violent, 42% (31/73) of nonviolent kids ate candy, but, 79% of non-candy eaters (158/200) became violent, as compared to 69% of candy eaters become violent. But you can reverse the apparent effect by increasing the base rate of candy eating to 50% say, and having 100 candy eaters. (hopefully I did not mess this up, but I think it's right). -------------------------- John W. Kulig Professor of Psychology Plymouth State University Plymouth NH 03264 -------------------------- ----- Original Message ----- From: "Dr. Martin Bourgeois" <mbour...@fgcu.edu> To: "Teaching in the Psychological Sciences (TIPS)" <tips@acsun.frostburg.edu> Sent: Thursday, October 1, 2009 12:21:53 PM GMT -05:00 US/Canada Eastern Subject: RE: [tips] Correlation example Good one. Here's what I find puzzling: British experts studied more than 17,000 children born in 1970 for about four decades. Of the children who ate candies or chocolates daily at age 10, 69 percent were later arrested for a violent offense by the age of 34. Of those who didn't have any violent clashes, 42 percent ate sweets daily. Am I missing something, or does this comparison make no sense? And 69% of the kids in the study who ate sweets were later arrested for a violent offense! And I thought we were a violent society on this side of the pond. From: Beth Benoit [beth.ben...@gmail.com] Sent: Thursday, October 01, 2009 12:04 PM To: Teaching in the Psychological Sciences (TIPS) Subject: [tips] Correlation example Here's an almost laughable example of "correlation is not causation" that some might find a good example for class. (Well, aren't they almost always laughable??) It's about a study that found that children who eat lots of candy are more likely to be arrested for violent behavior as adults. In all fairness, one researcher did try to encourage people to dig a little deeper: "Previous studies have found better nutrition leads to better behavior, in both children and adults. Moore said his results were not strong enough to recommend parents stop giving their children candies and chocolates. "This is an incredibly complex area," he said. "It's not fair to blame it on the candy." " But in my morning newspaper, neither that conclusion was posted, nor was the journal cited. Only the term "British researchers" was used. Here's the story: http://www.cbsnews.com/blogs/2009/10/01/crimesider/entry5355367.shtml Beth Benoit Granite State College Plymouth State University New Hampshire --- To make changes to your subscription contact: Bill Southerly (bsouthe...@frostburg.edu) --- To make changes to your subscription contact: Bill Southerly (bsouthe...@frostburg.edu) --- To make changes to your subscription contact: Bill Southerly (bsouthe...@frostburg.edu)