Sadly, I have some trouble figuring out when I should use permutation vs. combination (forget Baye’s formula, that is very tough for me)
For the question below, can someone walk me through the steps I should take? Thanks so much!
Q) A casual laborer has a 70% chance of finding work on each day that she reports to the day labor marketplace. What is the probability that she will work three days out of five?
A) 0.3087.
The question to ask yourself is whether the order of the items makes a difference or not. If it does, you use permutations; if it doesn’t, you use combinations.
Here, it doesn’t matter whether you say that our laborer worked Monday, Tuesday, and Friday, or Friday, Monday, and Tuesday, or Tuesday, Monday, and Friday, or . . . . Thus, you use combinations; this is a binomial distribution problem:
P(working 3 days) = 5C3[P(working)]³[1 – P(working)]²
= 10×0.7³×0.3²
= 0.3087
A simple note: binomial distributions use combinations, not permutations.
Absolutely perfect explanation - thanks S2000!
You’re too kind.
My pleasure.