A Brief Introduction to Statistics – Part 2 – Probability and Distributions

Probability concepts form the foundation for statistics.


A formal definition of probability:
The probability of an outcome is the proportion of times the outcome would
occur if we observed the random process an infinite number of times.
This is a corollary of the law of large numbers:
As more observations are collected, the proportion of occurrences with a particular outcome converges to the probability of that outcome.

Disjoint (mutually exclusive) events as events that cannot both happen at the same time. i.e. If A and B are disjoint, P(A and B) = 0
Complementary outcomes as mutually exclusive outcomes of the same random process whose probabilities add up to 1.
If A and B are complementary, P(A) + P(B) = 1

If A and B are independent, then having information on A does not tell us anything about B (and vice versa).
If A and B are disjoint, then knowing that A occurs tells us that B cannot occur (and vice versa).
Disjoint (mutually exclusive) events are always dependent since if one event occurs we know the other one cannot.
A probability distribution is a list of the possible outcomes with corresponding probabilities that satisfies three rules:

  1. The outcomes listed must be disjoint.
  2. Each probability must be between 0 and 1.
  3. The probabilities must total 1.

Using the general addition rule, the probability of union of events can be calculated.
If A and B are not mutually exclusive:
P(A or B) = P(A) + P(B) − P(A and B)
If A and B are mutually exclusive:
P(A or B) = P (A) + P (B), since for mutually exclusive events P(A and B) = 0

If a probability is based on a single variable, it is a marginal probability. The
probability of outcomes for two or more variables or processes is called a joint probability.
The conditional probability of the outcome of interest A given condition B is
computed as the following:
P(A|B) = P(A and B) / P(B)
Using the multiplication rule, the probability of intersection of events can be calculated.
If A and B are independent, P(A and B) = P(A) × P(B)
If A and B are dependent, P(A and B) = P(A|B) × P(B)
The rule of complements also holds when an event and its complement are conditioned on the same information:
P(A|B) = 1 − P(A’ |B) where A’ is the complement of A

Tree diagrams are a tool to organize outcomes and probabilities around the structure of the data. They are most useful when two or more processes occur in a sequence and each process is conditioned on its predecessors.
Bayes Theorem:
P(A1|B) = P(B|A1 )P(A1 ) / { P(B|A1 )P(A1 ) + P(B|A2 )P(A2 ) + · · · + P(B|Ak )P(Ak )} where A1, A2 , A3 , …, and Ak represent all possible outcomes of the first variable and P(B) is the outcome of second variable.
Drawing a tree diagram makes it easier to understand how two variables are connected. Use Bayes’ Theorem only when there are so many scenarios that drawing a tree diagram would be complex.

The standardized (Z) score of a data point as the number of standard deviations it is away from the mean: Z=(x−μ)/σ where μ=mean, and σ=standard deviation. If the tail (skew) is on the left (negative side), we have a negatively skewed distribution and a negative Z score of the median. In a right skewed distribution the Z score of the median is positive.

A random process or variable with a numerical outcome is called a random variable, denoted by a capital letter, e.g. X. The mean of the possible outcomes of X is called the expected value, denoted by E(X).

The most common distribution is the normal curve or normal distribution. Many variables are nearly normal, but none are exactly normal. Thus the normal distribution, while not perfect for any single problem, is very useful for a variety of problems. The normal distribution with mean 0 and
standard deviation 1 is called the standard normal distribution. An often-used thumb rule is the 68-95-99.5 rule, i.e. about 68%, 95%, and 99.7% of
observations fall within 1, 2, and 3, standard deviations of the mean in the normal distribution, respectively.

A Bernoulli random variable has exactly two possible outcomes, usually labeled success(1) and failure(0). If X is a random variable that takes value 1 with probability of success p and 0 with probability 1 − p, then X is a Bernoulli random variable with:

  • mean µ = p
  • and standard deviation σ = sqrt(p(1 − p))

The binomial distribution describes the probability of having exactly k
successes in n independent Bernoulli trials with probability of a success p.
The number of possible scenarios for obtaining k successes in n trials is given by the choose function (n choose k) = n!/(k!(n − k)!)
The probability of observing exactly k successes in n independent trials is given by:
(n choose k) p^k (1 − p)^(n−k) = (n!/(k!(n − k)!)) p^k (1-p)^(n-k)
Additionally, the mean, variance, and standard deviation of the number of observed successes are:
µ = np, σ^2 = np(1 − p), σ = sqrt(np(1-p))
To check if a random variable is binomial, use the following four conditions:

  1. The trials are independent.
  2. The number of trials, n, is fixed.
  3. Each trial outcome can be classified as a success or failure.
  4. The probability of a success, p, is the same for each trial.

The binomial formula is cumbersome when the sample size (n) is large, particularly when we consider a range of observations. In some cases we may use the normal distribution as an easier and faster way to estimate binomial probabilities. A thumb rule to use in such cases is to check the conditions:
np ≥ 10 and n(1−p) ≥ 10
The negative binomial distribution describes the probability of observing the k-th success on the n-th trial: (n-1 choose k-1) p^k(1-p)^(n-k) where p is the probability an individual trial is a success. All trials are assumed to be independent.

The Poisson distribution is often useful for estimating the number of rare events in a large population over a unit of time. Suppose we are watching for rare events and the number of observed events follows a Poisson distribution with rate λ.
P(observe k rare events) = λ^k e^-λ / k!
where k may take a value 0, 1, 2, and so on. e≈2.718, the base of natural logarithm.
A random variable may follow a Poisson distribution if the event being considered is rare, the population is large, and the events occur independently of each other.



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