What are the take-aways from the squash example?
Suppose that this particular study compared a large group of individuals who play squash regularly with those of an equal-sized group who get no exercise at all. Playing squash does provide a good cardiovascular workout. However, we also know that squash players tend to be affluent enough to belong to clubs with squash courts. Wealthy individuals may have great access to health care, which can also improve cardiovascular health. If our analysis is sloppy, we may attribute health benefits to playing squash when in fact the real benefit comes from being wealthy enough to play squash (in which case playing polo would also be associ ated with better heart health, even though the horse is doing most of the work). Or perhaps causality goes the other direction. Could having a healthy heart “cause” exercise? Yes. Individuals who are infirm, particularly those who have some incipient form of heart disease, will find it much harder to exercise. They will certainly be less likely to play squash regularly. Again, if the analysis is sloppy or oversimplified, the claim that exer cise is good for your health may simply reflect the fact that people who start out unhealthy find it hard to exercise. In this case, playing squash doesn't make anyone healthier; it merely separates the healthy from the unhealthy. There are so many potential regression pitfalls that I've devoted the next chapter to the most egregious errors. For now, we'll focus on what can go right. Regression analysis has the amazing capacity to isolate a statistical relationship that we care about, such as that between job con trol and heart disease, while taking into account other factors that might confuse the relationship. How exactly does this work? If we know that low-level British civil servants smoke more than their superiors, how can we discern which part of their poor cardiovascular health is due to their low-level jobs, and which part is due to the smoking? These two factors seem inextricably intertwined.What are the take-aways from the squash example?
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