Hello Data

Chapter 1
Math 215

Lending Club Loans

Six loans from the loan50 dataset
loan_amount interest_rate term grade state total_income homeownership
22000 10.90 60 B NJ 59000 rent
6000 9.92 36 B CA 60000 rent
25000 26.30 36 E SC 75000 mortgage
6000 9.92 36 B CA 75000 rent
25000 9.43 60 B OH 254000 mortgage
6400 9.92 36 B IN 67000 mortgage
  • Random sample of 50 loans made through Lending Club platform
  • A sample is a subset of a larger group (the population)
  • The sample consists of the 50 loans. The population is all loans made through the platform
Six loans from the loan50 dataset
loan_amount interest_rate term grade state total_income homeownership
22000 10.90 60 B NJ 59000 rent
6000 9.92 36 B CA 60000 rent
25000 26.30 36 E SC 75000 mortgage
6000 9.92 36 B CA 75000 rent
25000 9.43 60 B OH 254000 mortgage
6400 9.92 36 B IN 67000 mortgage
  • Data organized in a table called a data frame
Six loans from the loan50 dataset
loan_amount interest_rate term grade state total_income homeownership
22000 10.90 60 B NJ 59000 rent
6000 9.92 36 B CA 60000 rent
25000 26.30 36 E SC 75000 mortgage
6000 9.92 36 B CA 75000 rent
25000 9.43 60 B OH 254000 mortgage
6400 9.92 36 B IN 67000 mortgage

  • Each row represents a single case or observational unit

  • Each column represents a variable, corresponding to a loan characteristic. E.g.,

    • loan_amount (amount of loan in USD)
    • term (number of months of the loan)
    • grade (related to likelihood of being repaid)

Summary Statistics

  • A summary statistic is a single number that summarizes data from a sample
  • Mean loan amount ($17,083.00) is a summary statistic
  • Summary statistics can be organized in tables
grade mean interest rate
A 6.8
B 10.2
C 13.8
D 18.6
E 25.6

Association

  • If there is a relationship between two variables, we say that the variables are associated
  • Interest rate and loan grade appear to be associated
  • If there is no relationship between two variables, we say the variables are independent

Variable Types

  • A numerical variable takes on values that are described using numbers that make sense to add, subtract, average, etc
  • A categorical variable takes on values that indicate different levels or categories
Six loans from the loan50 dataset
loan_amount interest_rate term grade state total_income homeownership
22000 10.90 60 B NJ 59000 rent
6000 9.92 36 B CA 60000 rent
25000 26.30 36 E SC 75000 mortgage
6000 9.92 36 B CA 75000 rent
25000 9.43 60 B OH 254000 mortgage
6400 9.92 36 B IN 67000 mortgage

Loan data variable types:

Variable Type
loan_amount numerical
interest_rate numerical
term numerical
grade categorical
state categorical
total_income numerical
homeownership categorical

  • Numerical variables can be further broken down into

    • discrete: takes on discrete values (with jumps between consecutive values)
    • continuous: can take on any value within a range

  • Categorical variables can be further broken down into

    • ordinal: levels have a natural ordering
    • nominal: levels do not have a natural ordering

Variable types. From IMS1 Fig. 1.1.

Six loans from the loan50 dataset
loan_amount interest_rate term grade state total_income homeownership
22000 10.90 60 B NJ 59000 rent
6000 9.92 36 B CA 60000 rent
25000 26.30 36 E SC 75000 mortgage
6000 9.92 36 B CA 75000 rent
25000 9.43 60 B OH 254000 mortgage
6400 9.92 36 B IN 67000 mortgage

Loan data variable types:

Variable Type
loan_amount numerical, continuous
interest_rate numerical, continuous
term numerical, discrete
grade categorical, ordinal
state categorical, nominal
total_income numerical, continuous
homeownership categorical, nominal

Scatterplots

  • The relationship between two numerical variables can be visualized using a scatterplot

Scatterplot showing loan_amount vs. total_income.

Direction of association

  • Two numerical variables are said to have a positive association if the values of one variable tend to be higher when the values of the other variable are higher
  • Two numerical variables are said to have a negative association if the values of one variable tend to be lower when the values of the other variable are higher
  • Variables loan_amount and total_income appear to be positively associated

Example of A Negative Association

Scatterplot showing total_income vs. interest_rate.

Stents for Treating Strokes

First five patients
group outcome
control no event
treatment no event
treatment stroke
treatment no event
control no event
  • Researchers designed a study to study the effectiveness of stents in preventing strokes
  • 451 at-risk patients randomly assigned to receive stent (treatment) or not (control)
  • Outcome (“stroke” or “no event”) recorded after 365 days
First five patients
group outcome
control no event
treatment no event
treatment stroke
treatment no event
control no event
  • What are the cases?
  • What are the variables?
  • What are the variable types?
Summary of results for stent study
Group Stroke No Event Total
control 28 199 227
treatment 45 179 224
  • The proportion that had a stroke in the treatment group was 45/224 = 0.20
  • The proportion that had a stroke in the control group was 28/227 = 0.12
  • Does there appear to be an association between group and outcome?

Variable Roles

  • When we suspect one variable might causally affect another, we label the first variable the explanatory variable and the second the response variable.
  • We say that the explanatory variable affects the response variable
  • The terms explanatory and response can be used to describe variables where the response might be predicted using the explanatory even if there is no causal relationship.
  • In the stent study, the group (treatment or control) is the explanatory variable, and the outcome is the response variable

Experiment vs. Observational Study

  • An experiment is a study in which researchers researchers manipulate or assign the values of the explanatory variable
  • The stent study is an experiment, because the researchers assign patients to the treatment or control group
  • When cases are randomly assigned to groups, the study is called a randomized experiment
  • An observational study is a study without such manipulation. The cases are observed as they are
  • The Lending Club data are from an observational study