Probably question

Question

I have a 25% chance of something happening. How many times would I need to do it to have a reasonable chance of success?

Answer 1

This depends on what you consider to be "reasonable".

 

Probability of at least 1 success given n trials:

1-(1-0.25)^n

 

Let k be the desired level of certainty.

 

What you are doing is solving

 

1-(1-0.25)^n >= k for n.

 

For example, if you want to be 95% sure you'll have at least one success. k=0.95

 

1-(1-0.25)^n > 0.95

 

n >= 10.413. Rounded up (always up to guarantee the desired level of certainty) to 11. This means you need 11 trials to be 95% certain you'll get at least one success.

 

Another example, if you only need to be 80% sure

1-(1-0.25)^n > 0.80

n>=5.595, rounded up to 6. This means if you only need to be 80% certain, 6 trials is enough.

 

 

Answer 2

I'm not sure what you consider a "reasonable chance of success";

however, 25% = 25/100 = 1/4

 

This means the chance of the incident happening is 1 out of every 4.

If you do it 4 times, it should happen.