Let’s take the example of foot traffic in front of your house again. You have gathered enough data to established that groups of people pass by every 10 minutes. Using this data, you have made a reasonable assumption that the reason this cycle is happening is because there is a bus stop down the street and that every 10 minutes a bus stops and lets people off.
You want to bolster your theory so you dig deeper. You find a city bus schedule, and sure enough, there is a bus stop near your house, it does stop there every 10 minutes, and it is on a time schedule that matches your data. Now your bus stop theory is starting to get stronger and you can be more confident in predicting when the next group of people will walk by.
Then one day, the timing of people walking by is completely off. The next day everything is back on track again. Everything is going fine until, again, there is a day that is completely off. After this happens a few times you realize that every time the schedule is off, it is raining. Further observation confirms this, making the unpredictable more predictable. You know that it is very likely that the bus schedule will be off when it is raining. You know to plan for the schedule to be off when there is a chance of rain. You know that the schedule will be off with more regularity in the spring when you have observed that it rains more.
This example demonstrates that whenever you have rhythmic variation you probably have a cause. If you scatter iron filings on a desk you would expect them to be distributed at random. If you find them arranging themselves into a pattern, you can assume that some unknown force is present – perhaps a powerful magnet in the top drawer.
Similarly, if you find a pattern in the alternate thickness and thinness of tree rings or rock strata, or in the abundance of insects, or in the prices of common stocks, you can assume that there is a cause for such behavior. If you don’t know the cause, you don’t fully understand the behavior. But if you do know the cause, like with the rain and the bus schedule, you can better predict the rhythmic variations.