Comment by shash

13 hours ago

Is 12 enough of a sample size to make a statistical judgement? What if there were 20 more which didn’t have a loss of life? Is it then 1/30? What if there were 20 more?

The risk factor is calculated _per mission_ from what I understand. You can have three accidents in a row and nothing for decades but the risk itself can still be 1 in 30.

8 comments

shash

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the__alchemist  6 hours ago

Your point is fair and and important distinction. I think when estimating a risk factor though, this empirical data, while a low sample size, is a valuable statistic because it's empirical, and not that small of a sample size. Maybe going forward, we have 3 risk levels:

  - Historical. Low N as you say. (Even though each mission and spacecraft is different and they're spread out over time, there's value in this)
  - Bureaucrat number; absurdly low, but looks good to politicians etc
  - Engineering estimate

stetrain  13 hours ago

So the risk factor for Apollo could have actually been 1/1000 but they were just really unlucky?

  • shash  2 hours ago

    Yes, actually. This is similar to having a 100 year flood five years in a row. It doesn’t mean that the flood occurs only once in 100 years. _On average_ it’s 1/100 probability of occurring in any given year.

    But then, Apollo 1 was after all the first mission on the Saturn V. I think we should assess even its pre-launch risk much higher than the rest of them. Similarly Artemis II has a much higher risk than the subsequent ones will have.

    • stetrain  2 hours ago

      But we’re talking about the risk of a defined set of events that have concluded, not a prediction of the future.

      Of course Apollo would have likely had a better average if it had continued, but the risk of the Apollo program, as executed, included things like the first flight of the Saturn V.

      If the final empirical mortality result of the Artemis program is 1/30 or less, it will be better than Apollo in that statistic.

      A comparison of acceptable mortality is where this discussion began. If Apollo was acceptable at 1/12 (We did it, it was apparently acceptable as the program was not cancelled due to mortality rate) then an acceptable mortality of 1/30 is stronger than Apollo, not weaker.

  • d1sxeyes  7 hours ago

    If I toss a coin four times and it comes up heads three and tails once, it doesn’t mean that there’s a 75% chance that this coin lands heads up. Be careful about conflating risk factor and mortality rate.

    • stetrain  2 hours ago

      > If I toss a coin four times and it comes up heads three and tails once, it doesn’t mean that there’s a 75% chance that this coin lands heads up.

      No, but it means that to ensure that I do better on my next set of coin tosses I need to beat 3 in 4, not 1 in 2.

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