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Analysis > Tuesday, 12 January 2010 10:15:11 EST

On Predicting The Future

Keywords: statistics, predictions, religion, uncertainty, randomness

I often see predictions in the media attempting to predict the future.  Usually, these predictions are given out by the experts in a particular field, and we're lead to believe that they've actually got some sort of valuable insight into what the future holds.

What makes me crazy is that people seem to believe that these predictions have any kind of validity to them.  Some of them may be reliable, such as if the prediction is short term, or if it's based on unambiguous data whose properties are not generally subjected to random changes.

But this isn't the case for most predictions.  Randomness and uncertainty rule our lives, and to think that we can predict the long term future based on short term trends is to ignore this basic fact.  It's tempting to think that we can accurately predict the far future  based on what we observe happening today.  Unfortunately, predicting the future remains a very uncertain art.
Take for example the subject of religious affiliation in the United States.  In 2001, a report was released on religious demographics in the United States by the American Religious Identification Survey.  This report estimated that people who identified themselves as Christians made up about 79.8% of the population.

The difficulty came when people noticed that this was lower than the percentage of Christians estimate from 1990 of 88.3%.  When you do the math, and round it off, that's about one percentage point drop per year.  The message to a lot of people was obvious.  Christianity was in decline!  The estimates began coming in.  Sometime around 2030, Christianity was going to become a minority religion in the United States!

The problem, of course, is that there is no reason to believe that this was a consistent trend, or one that would continue on into the future.  If we were able to map out the changes in self reported religious affiliation throughout the 1990's, we would probably have not seen a steady decline of about 1% per year.  More likely, we would have seen an inconsistent graph that may have remained steady for several years, then taken a dip the next, perhaps climbing a little once in a while, but culminating at a low point in 2001. There should be no reason to think that the average decline is going to continue on forever.

In fact, we now have more updated numbers from 2007 released in a study by the PEW Forum On Religion & Public Life in February of 2008.  These more updated figures estimate the percentage of Christians in the United States to be 78.4%.  That's a decrease of only 1.4% in 6 years - a far cry from the 1% average per year estimated during the 1990's.  It appears that since 2001, Christianity in the US has only experienced an approximate 0.2% decline per year.  At this rate, Christianity won't become a minority religion in the US until sometime after 2149.

But, of course, there's no reason to expect that this new rate of 0.2% will remain steady either.  In fact, the 2007 number is so close to the 2001 number that the difference may not be statistically significant.  The lesson here may be that the percentage of Christians in America has levelled off and is showing no evidence of change.

If you play around with the concept of projecting current trends out into the future, you can come up with some decidedly silly results.  For example, a man may want to train for the 100 meter dash.  At first, he is able to run it in 35 seconds. A week later, he is able to do it in 30 seconds.  A week after that, he is proud of his progress of being able to run it in 25 seconds.  He does some calculations, and is excited to estimate that in 5 more weeks, he should be able to do the 25 meter dash in 0 seconds!  He is very pleased at the prospect of learning how to teleport to the finish line instantaneously.

What's more, he reasons that if he keeps on practising after that, he should be able to cross the finish line even before he starts.  He will have acquired the powers of teleportation and time travel simply by training for the 100 meter dash.  How exciting!

And, of course, there's the classic example of Elvis Impersonators.  In 1977, there were 37 Elvis Impersonators in the world.  By 1993, there were an estimated 48,000 Elvis Impersonators in the world.  It's 2010 now, and when you do the math, that means 1 out of every 3 people I see should soon be an Elvis impersonator.

I've got to say, I'm looking forward to that!


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