How Thinking in Probabilities Helps You Make Better Decisions

Charlie Jackson
Nov 9, 2018 · 4 min read


  • Assign probabilities to every aspect of a complex problem or choice
  • The probabilities do not need to be accurate
  • The process forces you to weigh up the different factors in a more neutral way, resulting in more accurate choices
  • Nothing can be 100% certain

We face difficult problems and decisions nearly every day. And it’s not always obvious what the most rational path is to take. A technique I’ve picked up from the amazing Harry Potter and the Methods of Rationality, is to assign probabilities to the different aspects of the issue, and use those to inform my conclusions. This is best illustrated with some examples:

Everyday Life — Work Estimates

As a programmer I get asked all the time to estimate how long particular tasks will take. There are many biases that distort the estimates we give, so I have turned to probabilities to improve the accuracy of these estimates.

Previously for a given task I may have instinctively replied:

Maybe it will take 2–3 days

Nowadays you’re more likely to hear:

I think there’s an 80% chance this will be done in 2 days, 15% chance within 3 days and 5% for any longer

Firstly to respond like this you need to think through what could go wrong, draw on previous experience of estimates going over and weigh it all up for this specific task. Something we may not do as often as we should.

Secondly, this conveys much more information to the manager, letting them know the level of uncertainty in the task (Not just to cover my ass if it goes over 3 days). It informs them on how likely it is that something might run over, so they can plan accordingly.

Considering Belief — The Simulation Argument

The ideas we believe in can dramatically alter the decisions we make every day and the direction in which we take our lives. So we should make sure we believe in things that make sense. Or just stop believing in ideas that can’t be reasonably proven.

Do we live in a computer simulation (like The Matrix). Emotionally I would love to believe this, as it’s a great comfort that when you die, it might just be the end of the simulation and you start a new one. So I wanted to see where I would rationally put the odds of us being in a simulation.

During my research I found a great post stating that one of the following must be true:

  1. If our civilisation got advanced enough to build a simulation as complex as our subjective experience, they would have no interest in building such a simulation
  2. It is not possible for a civilisation to get to a point where they could build such simulations
  3. There is an overwhelming chance you’re living in a simulation right now (> 99.99999%)

I found the argument interesting so I’ll use it here as an illustration of how I’d assign probabilities to an idea. I’ll leave it for another post to justify and criticise these points.

As one of these options must be true the total of all the odds needs to add up to 100%. Here’s how I would roughly distribute the following probabilities.

  1. 20% — Human motivations would need to change dramatically for this to be true. Sure, our motivations are likely to change if we merged with Artificial Intelligence (AI). But I still find such dramatic shifts less likely.
  2. 40% — It might not be possible to get to a point where we can build these simulations. As advanced civilisations may always end up wiping themselves out before reaching this stage (See the Fermi Paradox). Potential due to building the AI needed to make these simulations in the first place. It may also not be technologically possible.
  3. 40% — If the first 2 options are not true, it means that in our future we will reach a point where we could create billions of these simulations. And run many more simulations than there are humans. Given that you wouldn’t be able to distinguish our reality as a simulation or not, and there are billions going on. The odds of this reality being a simulation is billions to 1.

Turns out I have roughly put a 40% chance on it be near certainty that we’re living in a simulation, which is much lower than my instinctive feeling. Looking at it now my subconscious never weighed the first 2 options. This is a big shift in what I emotionally felt/hoped and highlights the value of assigning probabilities to your beliefs and not just blindly following.

Why this helps you think

The benefit of going through the probabilities doesn’t lie in coming up with accurate odds. It’s mainly about forcing yourself to rationally weigh up the different factors in the decision and not let our subconscious unfairly weigh some evidence more than others. Which we do every day thanks to our numerous biases.

It also helps you realise that nothing can be 100% certain. There is always a chance that a theory is wrong. 2 + 2 = 4? Maybe, what if all maths you’ve been taught has been a conspiracy, and everyone is in on it? Or we are in a simulation and the laws of physics inside the simulation have been made wrong on purpose as an experiment? Sure these are incredibly unlikely, but uncertainty exists for everything.

I’ve been applying this to both big and small decisions. The odds don’t need to be accurate, but the process should hopefully allow you to be less biased, more rational and have thought through some aspects you might have missed.


I’m writing to improve the way I think about and communicate ideas. Please give me some feedback on this post at And join my email newsletter here to keep up to date.

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