Monday, February 02, 2009

A Math Lesson

This is something that the previous post reminded me of and that I've been thinking about in general lately: the average experience can be different from the most common experience.

First some math. One of McKay's parents has 4 siblings. Actually, both of his parents have 4 siblings and the rest of the numbers are surprisingly the same also.

Each of those siblings have been married and have children and are done adding to their families. These 5 families (including McKay's) have, in order from smallest to largest: 3, 3, 4, 5, 8 children.

Now let's look at some very basic statistical figures.

Mean (average): 4.6
Median (middle number in the list from least to greatest): 4
Mode (most common number): 3
* See McKay's addition below

This sample size is small, so the "bell curve" is greatly skewed (as seen by the difference between the mean and median). This is a much simplified example, but it's pretty obvious that the average experience is different from the most common experience. While it might said that on average, McKay's aunts and uncles have 4.6, children, the mode is 3. Here, it is actually more common to have a smaller number of children than the average.

This is important to remember. Although it is typical for studies to have a greater sample size before coming to conclusions, it is still not guaranteed that the mean is the same as the mode- that the average experience is the same as the most common.

What does this mean for us? If it is determined that the average body temperature of an adult is 98.6 degrees Farenheit, does it mean that most adults have a body temperature of 98.6? Then there are other questions to ask: is that average determined just from men, or are women included (it's not atypical for men to be considered the "norm" and women are left out of studies)? What is the age range of these adults? How active are they? What climate do they live in? Do ethnic differences exist? etc.

When it is said that women gestate for 40 weeks, is that an average? Were the women studied first time moms? Were they of European descent or was there a variety of ethnicities included? Is that representative of the most common experience?

We should also question what is studied. We talk about 10 centimeters of cervical dilation; I searched and couldn't find a study that said that 10 centimeters came from measuring women's actual cervices. I did find a couple of places say it came from measuring the average diameter of a newborn's head (I'll have to find those; I forgot to bookmark them). Does the 10 centimeter standard take into account the amazing ability for a woman's body to stretch?

And then there is advice that, from what I can tell, is made up from concerns and not due to studies. We discussed at LLL last month the common advice to nurse a baby for 10 minutes on one breast and then to switch to the other for 10 minutes. Nowhere can I find numbers to substantiate this. I think this standard advice comes from the concern to make sure both breasts are stimulated to encourage supply. I really don't think the 10 minutes on both sides is an average or mode or anything related to the actual experience of mothers and babies.

Statistics and averages are convenient helpful to us for understanding the human experience, but it is important that when we see those results that we question what it means for us personally before we internalize those numbers and impose them on our own experience.

*McKay wanted me to mention the variance, "Doesn't mode affect the variance? Or at least, the variance would be higher if the mode is different from the mean." He's right, but I wanted to keep this simple. I learned about mean, median, and mode in fifth grade but didn't learn about variance until I took a college statistics class.


  1. That's really interesting!

  2. I had been wondering about the 10cm thing--for example, some women are larger and some are smaller, but do they really all dilate to 10cm? And certainly babies heads come in different sizes!

  3. I tried really hard to understand this but I got to the part with the numbers and my eyes glazed over. You'll have to explain in person I guess :)


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