Probability, Significance, Ranges

Until now I've been using probability without really defining it. Why talk about probability, when events either happen or they don't? A probability is an expression of how likely we think an event is. Not a great definition, using 'likely' (you might say it's a circular argument). Another definition is the fraction of times that an event would occur if you repeated the experiment forever. So if you flipped a coin a million times, you'd expect about half a million to be heads. The probability of heads is half a million divided by a million, or 0.5, or 50%.

  1. What is the probability of getting a six on a six sided die?

(Answer: All of the sides are equally likely, and there are six sides, so the answer is 1/6)

Now if we flip a coin a million times, we'd expect about a half a million heads. But what if we only flipped three or four times? Try it if you've got a coin. You probably won't get two heads and two tails. You certainly won't get a head and a half. That's the thing about probability and one of the pitfalls of statistics.

Let's say a tobacco company, call them Fill Morse, wanted to prove to the public that their cigarettes didn't cause cancer. Morse could get 5 smokers, find that one had cancer, and then say: "80% of smokers of Fill Morse cigarettes don't get cancer, new study says." This doesn't give a measure of the true probability of cancer incidence in smokers.

  1. What kind of study would be needed to come to a conclusion about cancer? What evidence would be needed, what comparisons would need to be made?

Statisticians talk about probability distributions. Especially, they talk about the normal distribution, or the (ready for it now) bell curve. One of the ideas in statistics is that if you have a huge group of samples and the variation between them is small (like if you measured the heights of north american men, say), it has a normal distribution. This means that the mean, median, and mode are the same value, and have the highest probability of occurring, with lower and lower probabilities as you go farther away from the mean (in other words, most men are 'average' height, and really really tall or really really short men are rare).

  1. Is the distribution of wealth normal? Is the distribution of income? (Hint: do most people have an 'average' income or wealth, if average is mean?)

The normal distribution looks like this—this one has a mean of zero, a standard deviation of ten.

But there are different kinds of normal distributions. The temperature in two cities might both have normal distributions, with similar mean temperatures, but the weather in one city, call it Varia, could be much more variable than in another city, call it Steadia.  The two distributions might look like this:

The point is that an average, by itself, doesn't say very much at all (as you knew from the discussion on averages above). That's why statisticians talk about the standard deviation and the significance.

Let's look at another example. Opinion polls are conducted all the time, which ask "do you support the bombing of helpless country x" or some such thing. Results are something like 60% no, 40% yes. And the fine print was something like "results are valid to within 1% 19 times out of 20".

The 1% is the deviation. The 19 times out of 20 is the significance.

What this means is that if you took another test, there would be 95% chance (19 out of 20) that the answer would come out between 59-61% no and 39-41% yes. In other words, the pollers are pretty confident that their sample is not biased and that it represents the population.

The standard deviation on the Stanford Binet IQ test is 3. The test assumes a normal distribution. The normal distribution means that the test told you you were worth 100 IQ points (idiotic to begin with, of course), there is only a 68% chance that your 'real' IQ falls between 97 and 103. There's a 95% chance that your IQ falls between 94 and 106, and as you increase the range, the chances go up that your value falls within the range.

These are not the greatest explanations of the phenomena but the point is that these figures, of standard deviation and significance, are what tell you how much an average can be trusted. They are hardly ever included with any statistics (except when the people giving them to you are confident, like in opinion polls). Which means you can hardly trust an average at all.



Next         Prev         Home