Lecture 2: Probability Distributions

Question 1

Frequency distributions

Answer 1

Summarise observed outcomes in a sample

Question 2

Contingency table

Answer 2

Used to describe the joint frequency distribution, and possibly relationship, between two categorical variables

Question 3

The standard normal distribution (Z-distribution)

Answer 3

A standardised version of the normal distribution, rescaled to have a mean of 0 and a standard deviation of 1

Question 4

Discrete random variables

Answer 4

Variables with a finite or countable number of possible outcomes

Question 5

Continuous random variables

Answer 5

Variables with an infinite number of possible outcomes

Question 6

Indexing a contingency table

Answer 6

Telling other people which cell in the contingency table to look at

Row = i, column = j, frequency = f

E.g., look at index i = 2 means to look at row 2

Question 7

Joint frequency distribution

Answer 7

Frequency of a combination of two (or more) variables
E.g., the frequency of Dutch students with a tattoo

Question 8

Marginal frequency distribution

Answer 8

Looks at multiple variables and how often they occur
E.g., “tattoo” and “Dutch”

Question 9

Marginal probability distribution

Answer 9

Divide marginal totals by the global totals (i.e., the entire sample)

If the entire sample is 74 and 21 people of this sample are non-Dutch, then the probability of someone being non-Dutch is 21/74 = 0.28

Question 10

Conditional probability distribution

Answer 10

Divide the row- or column frequencies by the total of the row/column that we are interested in
E.g., what is the conditional probability of having a tattoo for international students? P (Tattoo | Dutch => chance of having a tattoo, given that you are Dutch)

Question 11

Joint probability distribution

Answer 11

Dividing the cell frequency by the global total

E.g.., what is the joint probability of someone being Dutch and having a tattoo?
P (Dutch ∩ Tattoo)
The symbol ∩ refers to ‘and’