Question

Here is the beginning of an article from the YAHOO Health News web site on January 7, 1998: “Wednesday January 7, 1998 For Yahoo News by Reuters Daily Two–Mile Walk Halves Death Risk NEW YORK (Reuters) — Walking two miles or more per day can cut the overall risk of dying in half, according to a new study. It also reduces the risk of dying from cancer — and appears to cut the risk of death due to cardiovascular diseases, US researchers report. Between 1980 and 1982, multicenter researchers in the Honolulu Heart Program studied 707 nonsmoking, retired men, aged 61 to 81 years, and collected mortality data on these men over the following 12 years. During the study, 208 of the men died. The study results show that while 43.1% of men who walked less than one mile per day died, only half this figure – 21.5% – of the men who walked more than two miles per day died.”

(a) Is the research described in the article an experiment or an observational study?

(b) List two different possible confounding factors that are likely to have an effect on the outcome of this study. Carefully explain exactly why you think these are confounding factors.

(c) Based on the description in the body of this article, is the headline (“Daily Two–Mile Walk Halves Death Risk”) justified? Yes or No? Circle your answer and give a brief explanation.

Answer 1

(a)

The research described in the article is an observational study because the researchers did not control any variable but just observed what has happened over the years to draw conclusions.

(b)

The two different possible confounding factors that are likely to have an effect on the outcome of this study are:

1. Health and fitness including presence of any diseases and

2. Sudden deaths due to accidents, electric shocks, murders, suicides, falling down the stairs, etc,.

I think these are confounding factors because if these factors were considered then the conclusion might be that the deaths were due to these factors and walking had no influence or little influence on deaths.

(c)

No.

Based on the description in the body of this article, the headline “Daily Two–Mile Walk Halves Death Risk” is not justified because 43.1% + 21.5% is only 64.6% but not 100% or close to 100%.

It's because, out of 707 persons, deaths of who walked 2 miles or more is separated which is 21.5% of 707 persons(152) and deaths of who walked less than 2 miles is separated which is 43.1% of 707 persons(305). Then what about the remaining living persons (707-152-305 =250) - were they all walked more than 2 miles? What if all or most of these 250 members walked less than 2 miles a day and are still living which means walking less than 2 miles a day reduced the risk of death because these 250 members constitute a considerable percentage, 35.4% (100-43.1-21.5) =35.4) of 707 members.

Thus, it is necessary to consider the walking scenario of the living persons before concluding.

If researchers waited till all or most (>90%) of 707 members died and record the percentage of deaths of who walked 2 miles or more and less than 2 miles, then their conclusions would be correct but only based on the description of this study, that is, assuming no confounding effect. For example 67% of persons who walked less than 2 miles a day died and half of it, i.e., 33% of persons who walked 2 miles or more a day died, then the headline “Daily Two–Mile Walk Halves Death Risk” would be justified.