Interview Q&A / Data Science

1. How do you calculate conditional probability from a two-way table?

   P(A given B) is the count of A-and-B divided by the count of B. You restrict attention to the rows or columns where B is true and compute the share there. P(A given B) is not the same as P(B given A).

   BeginnerQuick answer1 min read
2. How do you find the probability of at least one success?

   Use the complement rule: P(at least one) = 1 - P(none). Computing the chance of nothing happening is much easier than adding up every way that something happens.

   BeginnerQuick answer1 min read
3. What is expected value, and how do you use it to judge a game or decision?

   Expected value is the probability-weighted average of all outcomes, the long-run average result of repeating a situation many times. A negative expected value means you lose on average, even if you sometimes win.

   BeginnerQuick answer1 min read
4. What is the difference between independent and mutually exclusive events?

   Independent events do not affect each other: P(A and B) = P(A) x P(B). Mutually exclusive events cannot happen together: P(A and B) = 0. Two events with non-zero probability cannot be both.

   BeginnerQuick answer1 min read
5. What is the difference between permutations and combinations?

   Permutations count arrangements where order matters; combinations count selections where order does not. Choosing r from n gives nPr = n!/(n-r)! ordered ways and nCr = n!/(r!(n-r)!) unordered ways.

   BeginnerQuick answer1 min read
6. When do you use the binomial distribution?

   Use it to count successes in a fixed number of independent trials, each with the same probability of success. The probability of exactly k successes in n trials is C(n,k) p^k (1-p)^(n-k).

   BeginnerQuick answer1 min read