Sandbox

Walking and Dementia

As per usual, eye-popping headlines in the press and on the web can be misleading:

• MSNBC: “One way to ward off Alzheimer's: Take a hike”
• ABC News: “Walking May Keep Brain From Shrinking in Old Age”
• Punjab Newsline: “Walking helps retain memory in old age”
• Oregon Live: “Prime of life: Forget thinner thighs, new study shows walking protects brain size, saves memory”
• Independent: “Walking can save your memory”
• Thirdage: “Alzheimer’s Disease Can be Offset by Walking”

The research article on which this news coverage is based--Erickson, et al, “Physical activity predicts gray matter volume in late adulthood: The Cardiovascular Health Study”, Neurology, first published on October 13, 2010--has more modest proclamations.

We cannot conclude a causal association between PA [physical activity] and GM [gray matter] volume.

…given the observational nature of the study, we are unable to conclude that PA causes greater GM volume.

Greater amounts of walking are associated with greater gray matter volume, which is in turn associated with a reduced risk of cognitive impairment.

The study began in 1998-1990 with 1479 elderly subjects who “were free of dementia and mild cognitive impairment (MCI).” These subjects had their physical activity assessed at that time. In the nine years of follow-up, MRI’s were performed; this resulted in “299 cognitively normal subjects.” After four further years of follow-up, “183 remained cognitively normal in 2002-03” and “116 diagnosed MCI or dementia in 2002-03.” The authors chose to disaggregate the 299 into four groups depending on the number of blocks walked per week, 0-12, 13-24, 25-70 and 72-300 with 72 blocks being equated roughly to between 6 and 9 miles. The study claims that the last group had less gray matter shrinkage than any of the other three with no significant difference among the others.

Discussion
1. The data for the number of blocks walked per week were obtained by asking the participants. Why is this a problem?
2. Why was tennis rejected as a measure of physical activity?
3. The underlying belief is that walking has an effect on brain health. Defend the notion that brain health has an effect on the desire to walk. The authors put it this way: “there remains a possibility that reduced amounts of walking is a result of ill health and that ill health leads to both reduced amounts of walking and GM volume loss.”
4. The original data were obtained in 1989-1990 and the final data occurred in 2002-03; the paper has ten authors and was published in 2010. What does this say about the difficulties of studying humans as opposed to fruit flies?
5. According to Science 20: “The researchers found that those who walked the most cut their risk of developing memory problems in half.” Why is this “risk” a relative risk and not an absolute risk?
6. The study does not show the percentage of cognitive impairment in each of the four walking categories. Why would this be helpful?
7. The statement found in the Conclusion, Greater amounts of walking are associated with greater gray matter volume, which is in turn associated with a reduced risk of cognitive impairment. hints, via the concept of transitivity, that greater amounts of walking is associated with a reduced risk of cognitive impairment. That is, if A is related to B and B is related to C, then A is related to C. However, see Langford for a discussion of why this is not true in general. In particular, here is one of its counter examples:

X Y Z 4 201 15 6 165 30 1 145 28 1 150 41 0 160 18 2 113 5 5 140 7 4 147 16 0 83 15 0 108 16

Use a stats package to show that even though the correlation between X and Y is positive, and the correlation between Y and Z is positive, but the correlation between X and Z is negative.

Submitted by Paul Alper

Forecasting Campaign 2010

T'he Wall Street Journal
October 23, 2010
The Numbers Guy (Carl Blank)] says.

My print column this week examines the tough task facing forecasters of the House of Representatives races this fall. Political scientists, veteran pundits and number crunchers have advanced the field considerably from several decades ago, thanks to freely available poll data and advanced computer models, but they still run up against inherent challenges of quantifying public mood and the small number of observations: There is only one midterm election every four years, after all.

Of course it is interesting to see how the predictions were

freakonomics.blogs.nytimes.

the prediction of the model is that the Democrats will end up with 199 seats, or a loss of 56 seats.

Submitted by Laurie Snell
To be continued