The Doomsday Calculation

 

Consider answering the following questions:

How much longer will humans exist?

How long will a marriage last?

How much longer will that building be standing?

William Poundstone explains a simple approach to answering such questions in his book The Doomsday Calculation: How a Formula That Predicts the Future Is Transforming Everything We Know About Life and the Universe.

The Doomsday Calculation approach uses the Copernican principal which states that we are not in a special place (or time). Let’s take an example of you meet two couples for the first time at an event. The first couple has been together for a year, second couple for ten years. If you had no other knowledge of their relationships and you had to predict which couple would more likely be still together in a year, most people who probably pick the second couple. Your answer would be the same if you had to answer the question for being still together in five years or more. The first couple may be perfectly matched, but only given the evidence of the current length of their relationships, the second couple has demonstrated more longevity.

The Copernican principal states our observation of the couple is at no special time. In meeting the second couple who has been together ten years, we could be meeting them near the beginning of 70+ year marriage or we could be meeting them a year from a breakup, Let’s breakup the full (unknown) length of the relationship into quarters. We have a 25% chance of meeting them in the first quarter, a  50% chance of meeting them in the middle two quarters, and a 25% chance of meeting them in the final quarter.

The middle 50% is the range we’re interested in. If your meeting with them happens at the low end of this range, 25% of the length of their relationship has been ten years and they have another 75% or thirty years left. On the other hand, if ten years is the 75%, they only have another 25% left, which in this case is three years and four months remaining. With those two extremes evaluated, we can state we are 50% confident the relationship will last between three years and four months to thirty additional years.

Perhaps you want to state your range with a higher confidence level. That will mean widening the interval. Moving to 75% confidence, the interval becomes approximately a year and a half to seventy more additional years.

Poundstone gave a brief summary of the doomsday calculation with additional examples in: Doomsday argument: the end of humanity, predicted by a math equation - Vox

 

William Poundstone is the author of 16 books, including The Doomsday Calculation: How a Formula That Predicts the Future Is Transforming Everything We Know About Life and the Universe.

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