Probability of X happening after n tries

[jolee934]

Hi all.
This isn't a homework problem but I wanted to confirm something for a real life scenario.

Let's say the probability of something happening is 0.00005%, number of trials is 20k, and we want to see if an event will happen AT LEAST once.
I used this calculator.. https://stattrek.com/online-calculator/binomial.aspx

The answer I get is 0.99% (Cumulative probability of X>=1) Is that correct? I'm being told that I should multiply the answer by 100 but I don't see why. The input for "Probability of success on a single trial" is 0.0000005 right? (since the actual probability is 0.00005%, it should be divided by 100.. and i already multipled the final answer by 100, which comes to 0.99)

 

[Cubist]

The answer I get is 0.99% (Cumulative probability of X>=1) Is that correct?

0.995% is correct if we round the answer to 3dp (or with a few more dp 0.99501687%). If you require 2dp then 1.00% would be correct.

I'm being told that I should multiply the answer by 100 but I don't see why. The input for "Probability of success on a single trial" is 0.0000005 right? (since the actual probability is 0.00005%, it should be divided by 100.. and i already multipled the final answer by 100, which comes to 0.99)

You don't need to multiply the 0.995% by 100.

To check your method you could use some numbers that are more intuitive, like a 50% chance of the event happening and 2 trials. You'd expect the event to happen at least once with a probability of 75%.