The Nature of Correlation – Part II

Since we now understand some basics about correlation I think it would be interesting to try and understand “How do we initially hypothesize a connection between variables?”  If we analyze something simple like Newton’s second law which says that the force exerted by an object is directly related to its mass and acceleration.  How might such a relationship have been devised initially?  Well this is something that can be easily observed.  You push an object like a


cart and it accelerates.  If you push it harder, it accelerates faster.  If the cart is full of objects it becomes heavier and requires more force to get the same acceleration.  Such observations are the basis for an experiment which can show us the nature of the relationship.  Our experiment might end up only being mostly successful as we might be confused about why the force might change depending on the surface across which we move the object.  Until we understand friction we might not be fully aware of the other forces that are working against us when we make our measurements.  Newton was able to also explain the frictional force as well, but the fact that the Earth is turning on its axis revealed Newton’s second law to be incomplete for Earth and only true in a reference frame that is not in motion.   Newton’s description of force came first and it was only through further experimentation and testing that we came to understand the limitations associated with his relationship between force, mass and acceleration.  So the basis for a correlation comes through a lot of trial and error after some initial observations.  Proper application of the scientific method (adequate sampling in particular) along with result repeatability can demonstrate the correlation between two variables.

Finally it’s important to understand the complexity of relationships that exist.  Just like the force is dependent on the mass and acceleration, most things in this world are not as simple as one cause and one effect.  Most things are complex systems in that one variable may change as a result of several other variables.   The global average surface temperature is a function of the amount of solar radiation hitting the Earth, the concentration and location of greenhouse gases in the atmosphere, the amount of geothermal energy released at the surface, and the amount of energy released through radioactivity.  The last two tend to be fairly small.  Solar radiation is the most important factor then followed by the greenhouse effect.  So if we want to look at how CO2 varies with global temperature, we are never going to get a perfect correlation, but we are going to see a correlation.  And if we understand the role the sun plays in heating, we can easily delineate between which part of the heating is due to changes in solar radiation, and which part of the heating is due to changes in greenhouse gases.  Just like with our example of force, we can determine whether that force is because of a light object traveling at a fast speed or whether it is a heavy object traveling at a low speed.  Our knowledge of the relationship allows us to make that determination.

In social sciences the variables impacting a system can be numerous.  As I’ve argued before gun control is an extremely complex issue.  The number of gun deaths is dependent on the types of laws we have, the number of guns available, quality of mental health care, attitude towards mental health care, income inequality, education (both general education and education about use of guns), the role of the media and politicians fear mongering, culture attitude towards violence and death, and probably more than that.  With these types of issues it’s easy to point to all the other things that could be causes to try to show that changing one variable isn’t going to have an impact.  But neither side is completely valid here, because the argument should really be about the factors that are more important and which ones are less important.   Just because one variable is more important however doesn’t eliminate all other variables from having an influence.  Just like coalition governments where the dominant party can lose power if the other parties combined outweigh them in votes, a dominant variable in a complex system may be outweighed by the combined importance of the other variables.  For instance when it comes to the greenhouse effect, carbon dioxide is not the only important greenhouse gas.  Numerous greenhouse gasses like CFC’s, nitrous oxide, ozone, and methane are released as pollutants and if industrialization continues at the pace it is at, the combined impact of those other gasses may become similar to the impact of CO2, even though individually those gases have a very small effect.  It is important to understand all the variables that are involved and address them, especially when harm is being caused to people, because even a small variable that we can fix might reduce that harm.

As the complexity of a system increases the direct correlations between one variable and another generally decrease.  A correlation of 0.2 might be significant if there are many variables all impacting the state of a system, especially if all those variables might be of similar importance.  People like to keep relationships simple, but by doing so fail to solve problems that are usually far more complex.  This is also why complex systems are some of the easiest for those who don’t really understand it to mislead others.  Climate change is a great example.  A change to the climate system depends on many factors and thus makes it easy for someone to try and emphasize one part to make their argument.  Like the oft used “Carbon dioxide is necessary for plants to grow, how can more be a bad thing?” This ignores the role carbon dioxide plays in the greenhouse effect, ocean acidification, and what happens when plants decompose.

Happiness is often brought about by the simple things in life, but it is also important to remember that there are lots of things happening out there that aren’t so simple.  We are a part of a complex universe.  Part of why we continue to survive better is that we continue to breakdown the complexities of the university into things that we can understand.  Also remember that just because things are complex doesn’t mean that there aren’t those who truly understand the problem and that with patience and effort you can too if you choose.


5 thoughts on “The Nature of Correlation – Part II

  1. Heh, seems like a great idea, I’ll toss in a few bucks for an ink cartridge 🙂 Teaching creationists how to think is a worthwhile cause!

    Many great points there Swarn, I like the place where you are coming from. It is one thing to understand these concepts, it is yet another to explain them well. Many would do well to read and understand your Correlation Series.


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