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Causes and Effects Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 12, 2011

Y causes X

Imagine that the horizontal axis in Fig. 7-4 represents 12 different groups of people in a medical research survey.

Causes and Effects

Each hash mark on the horizontal axis represents one group. Plot X is a point-to-point graph of the relative number of fatal strokes in a given year for the people in each of the 12 groups; plot Y is a point-to-point graph of the relative average blood pressure levels of the people in the 12 groups during the same year. (These are hypothetical graphs, not based on any real historical experiments, but a real-life survey might come up with results something like this. Medical research has shown a correlation between blood pressure and the frequency of fatal strokes.)

Is there a cause-and-effect relationship between the value of X and the value of Y here? Most doctors would answer with a qualified yes: variations in Y cause the observed variations in X (Fig. 7-5B). Simply put: high blood pressure can cause fatal strokes, in the sense that, if all other factors are equal, a person with high blood pressure is more likely to have a fatal stroke than a person identical in every other respect, but with normal blood pressure.

Causes and Effects

Fig. 7-5B. At B, Y causes X.

What about the reverse argument? Can fatal strokes cause high blood pressure (X causes Y)? No. That's clearly absurd.

Complications

Experts in meteorology and medical science who read this may, at this point, be getting a little nervous. Aren't the above scenarios oversimplified? Yes, they are. The cause-and-effect relationships described aren't ''pure.'' In real life, ''pure'' cause–effect events, where there is one certain cause and one inevitable effect, rarely occur.

The brightness of the sunshine is not, all by itself, the only cause-and-effect factor in the temperature during the course of a day. A nearby lake or ocean, the wind direction and speed, and the passage of a weather front can all have an effect on the temperature at any given location. We've all seen the weather clear and brighten, along with an abrupt drop in temperature, when a strong front passes by. The sun comes out, and it gets cold. That defies the notion that bright sun causes things to heat up, even though the notion, in its ''pure'' form where all other factors are equal, is valid. The problem is that other factors are not always equal!

In regards to the blood-pressure-versus-stroke relationship, there are numerous other factors involved, and scientists aren't sure they know them all yet. New discoveries are constantly being made in this field. Examples of other factors that might play cause-and-effect roles in the occurrence of fatal strokes include nutrition, stress, cholesterol level, body fat index, presence or absence of diabetes, age, and heredity. A cause-and-effect relationship (Y causes X) exists, but it is not ''pure.''

D causes both X and Y

Now suppose that the horizontal axis in Fig. 7-4 represents 12 different groups of people in another medical research survey. Again, each hash mark on the horizontal axis represents one group. Plot X is a point-to-point graph of the relative number of heart attacks in a given year for the people in each of the 12 groups; plot Y is a point-to-point graph of the relative average blood cholesterol levels of the people in the 12 groups during the same year. As in the stroke scenario, these are hypothetical graphs. But they're plausible. Medicine has shown a correlation between blood cholesterol and the frequency of heart attacks.

Before I make enemies in the medical profession, let me say that the purpose of this discussion is not to resolve the cholesterol-versus-heart-disease issue, but to illustrate complex cause–effect relationships. It's easier to understand a discussion about real-life factors than to leave things entirely generic. I do not have the answer to the cholesterol-versus-heart-disease riddle. If I did, I'd be writing a different book.

When scientists first began to examine the hearts of people who died of heart attacks in the early and middle 1900s, they found ''lumps'' called plaques in the arteries. It was theorized that plaques caused the blood flow to slow down, contributing to clots that eventually cut off the blood to part of the heart, causing tissue death. The plaques were found to contain cholesterol. Evidently, cholesterol could accumulate inside the arteries. Consulting data showing a correlation between blood cholesterol levels and heart attacks, scientists got the idea that if the level of cholesterol in the blood could be reduced, the likelihood of a heart attack would also go down. The theory was that fewer or smaller plaques would form, reducing the chances of clot formation that could obstruct an artery. At least, such was the hope.

The obvious first point of attack was to tell heart patients to reduce the amount of cholesterol in their food, hoping that this change in eating behavior would cause blood cholesterol levels to go down. In many cases, a low-cholesterol diet did bring down blood cholesterol levels. (Later, special drugs were developed that had the same effect.) Studies continue along these lines. It is becoming apparent that reducing the amount of cholesterol in the diet, mainly by substituting fruits, vegetables, and whole grains for cholesterol-rich foods, can reduce the levels of cholesterol in the blood. This type of dietary improvement can apparently also reduce the likelihood that a person will have a heart attack in the next year, or two, or three. There's more than mere correlation going on here. There's causation, too. But how much causation is there? And in what directions does it operate?

Let's call the amount of dietary cholesterol ''factor D.'' According to current popular medical theory, there is a cause-and-effect relation between this factor and both X and Y. Some studies have indicated that, all other things being equal, people who eat lots of cholesterol-rich foods have more heart attacks than people whose diets are cholesterol-lean. The scenario is shown in Fig. 7-5C. There is a cause-and-effect relation between factor D (the amount of cholesterol in the diet) and factor X (the number of heart attacks); there is also a cause-and-effect relation between factor D and factor Y (the average blood cholesterol level). But most scientists would agree that it's an oversimplification to say this represents the whole picture. If you become a strict vegetarian and avoid cholesterol-containing foods altogether, there's no guarantee that you'll never have a heart attack. If you eat steak and eggs every day for breakfast, it doesn't mean that you are doomed to have a heart attack. The cause-and-effect relationship exists, but it's not ''pure,'' and it's not absolute.

Causes and Effects

Fig. 7-5C. At C, D causes both X and Y.

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