Tuesday, July 22, 2008

What is "normal"?

Homebirth advocates like to pretend that almost anything that happens is "normal" simply by virtue of the fact that it happened. Are you still pregnant 3 weeks after your due date? Must be normal, since it happened. Are you in labor and stuck at 8 cm for the past 6 hours? Must be normal, since it has happened to some women in the past, and a few have even gone on to deliver live babies.

The corollary of the homebirth fantasy that almost everything is "normal" is the conviction that medical definitions of "normal" are utterly arbitrary and exist merely for the convenience of doctors. Nothing could be further from the truth. Often, "normal" is based on knowing the outcomes from previous experience. We can confidently say that having an Apgar score of 1 at 5 minutes of life is not normal, because babies who have Apgar scores of 1 at 5 minutes always have serious medical problems of one kind or another.

Sometimes "normal" is defined as a range. That is not an accident, and it does not mean that a range was chosen arbitrarily. A normal range in medicine is almost always based on a basic and widely accepted form of statistical analysis, the standard deviation.

There is an excellent simple explanation of standard deviation on SensibleTalk.com. It is written for journalists who have no background in statistics:
Let's say you are writing a story about nutrition. You need to look at people's typical daily calorie consumption. Like most data, the numbers for people's typical consumption probably will turn out to be normally distributed. That is, for most people, their consumption will be close to the mean, while fewer people eat a lot more or a lot less than the mean.

When you think about it, that's just common sense. Not that many people are getting by on a single serving of kelp and rice. Or on eight meals of steak and milkshakes. Most people lie somewhere in between.
When you graph the data with calories on the x-axis and numbers of people on the y-axis, you will get a bell shaped curve.

The curve is a graphical representation of all the possible things that can happen. The important point, though, is that every possible thing that can happen is not necessarily normal. How do we tell the difference between normal and abnormal? We start by calculating the standard deviation. The formula for calculating the standard deviation is complicated, but the result is relatively simple to understand. The standard deviation is a reflection of distribution of all possible outcomes.

Mathematically, one standard deviation on each side of the mean (the average) encompasses 68% of individuals. Two standard deviations encompasses 95% of individuals. Therefore, only 5% of individuals will be outside of two standard deviations from the mean. This is always true, regardless of whether the bell curve is tall and narrow or short and extended. "Normal" is usual defined as within two standard deviations. That means that "normal" is a range, but the range is hardly arbitrary. It reflects the actual distribution of results among large populations of human beings.

So when we look at how long a first labor lasts, for example, we can graph the labors of large numbers of women and we will get a bell curve. Ninety-five percent of women will fall within two standard deviations of the mean. It is only those women who are outside of two standard deviations that are considered abnormal. That does not mean that a woman whose labor is lasting longer than two standard deviations from the mean cannot possibly have a vaginal delivery, but it does mean that a woman whose labor is lasting longer than two standard deviations from the mean is very unlikely to have a vaginal delivery.

The bottom line is this: defining normal as a range is not arbitrary. It is a reflection of what we know about human variation. The range of normal ALREADY accounts for most of human variation. Anything that lies outside the range of normal is very unlikely to be normal.


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