Regular readers will recognize DAV as a regular contributer whose humorous voice of reason has kept many conversations on target. Today he presents ideas to spur a discussion of causality, a topic which, strangely enough, is often neglected in statistics.
I’ve been doing some thinking lately about the meaning of the word “cause.” I don’t mean in the sense of an association of individuals enjoined in a common endeavor. I rather mean “cause” as in “cause and effect.” Everybody has their own ideas of what a “cause” is. I met someone a while ago that insisted on varying flavors of cause such a Associate, Secondary or Joint none of which should properly be labeled Cause. It struck me as overly precise and much akin to saying hamburger is not meat—steak is.
One thing is clear though. Whatever a “cause” is it must precede the effect. Or does it? If there’s room we’ll come back to that.
When we say “X causes Y” what do we really mean? We might say a trigger is the cause of a gun firing but the trigger actually sets off a chain of events leading to the gun firing. It’s not THE cause in the sense that it is the ultimate event prior to the discharge. So why do we say the trigger is the cause and ignore the firing pin, primer and propellant?
Part of the reason may be that the trigger directly interfaces with the operator. Saying the trigger is the cause carries a number of tacit assumptions: the gun is working properly; it was loaded; the ammunition was built correctly; and more. Oddly, the safety, or more precisely its setting, is also part of the event chain. That implies inhibitors are also causes if “cause” is everything in the event chain.
A major notion is time sequence. X comes before Y therefore Y cannot be the cause of X. Establishing time order can be tricky in cyclical scenarios. There’s the notion of volition. Guns don’t (normally) fire by themselves. A possible reason why the trigger is labeled “the cause.” There’s also the notion of separation meaning the closeness (in time or number of events) to the effect but apparently the notion of volition trumps that. It does for guns, anyway.
I think that people, whether they realize it or not, actually mean cause is something that can be used to predict an effect. In other words, they rely on the dependence between the two. But not the dependence alone. It’s also the independence. Causes are (normally) independent of their effects so effect (normally) can’t be used to predict the cause. But there are times when determining precedence becomes a chicken and egg problem. For example a drug user takes a drug for its effect. The resulting effect could lead to more drug use. Sometimes, the merely the anticipation of a drug’s effect leads to its use. In that sense, the effect is one of its own causes. Sometimes, time information may not exist.
So, in the long run, cause seems to be determined by correlation or its absence. The common adage about correlation and causality really only applies to two variable situations. An experiment is just an active way to introduce more variables (hopefully in a controlled manor) into the mix.
There’s also the idea of level of predictability. In the comments of my previous post, there was a brief discussion on whether a slap could result in a fatal leap to a conclusion. Well, it’s a cause in the sense that it is part of the chain leading to the leap. But our sensibilities and experience tell us the slap is an outlier in this case.
A lot of this comes from my use of Bayes Nets. A Bayes net is a graphical model of the joint probability of all of the variables. As it turns out, any given node within the network is insulated from the others by the surrounding variables. This is called a Markov Blanket. A node can be completely described by this blanket which includes its parents, its children and the parents of its children. In a Bayes Net, the parents are causes and the children are their effects. Interestingly, the idea of the Markov Blanket says that events can be predicted by their causes.
I’ve run out of room. I really don’t have a point here. Just some ramblings to think about.