Lecture 7

Charts

DATA 8

Fall 2017

Slides created by John DeNero (denero@berkeley.edu) and Ani Adhikari (adhikari@berkeley.edu)

Announcements

Census Continued

(Demo)

Data Visualization

Discussion Question

Which of the following questions can be answered by this chart?

Among survey responders...

• What proportion did not use their phone for online banking?
• What proportion either used their phone for online banking or to �look up real estate listings?
• Did everyone use their phone for at least one of these activities?
• Did anyone use their phone for both online banking and real estate?

Pew research center, 2014

Area Principle

Areas should be proportional to the values they represent

Example from Tian Zheng

Numerical Data

(Demo)

How Do You Generate This Chart?

Top 10 highest grossing movies

How long ago each one was released

Types of Data

All values in a column should be both the same type and be comparable to each other in some way

• Numerical — Each value is from a numerical scale
• Numerical measurements are ordered
• Differences are meaningful
• Categorical — Each value is from a fixed inventory
• May or may not have an ordering
• Categories are the same or different

“Numerical” Data

Just because the values are numbers, doesn’t mean the variable is numerical

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• Census example had numerical SEX code (0, 1, and 2)

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• It doesn’t make sense to perform arithmetic on these “numbers”, e.g. 1 - 0 or (0+1+2)/3 are nonsense here

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• The variable SEX is still categorical, even though numbers were used for the categories

Terminology

• Individuals: those whose features are recorded
• Variables: features; these vary across individuals
• Variables have different values
• Values can be numerical, or categorical, or of many other types
• Distribution: For each different value of the variable, the frequency of individuals that have that value
• Frequency is measured in counts. Later we will use proportions or percents.

Plotting Two Numerical Variables

Scatter plot: scatter

Line graph: plot

Categorical Data

(Demo)

Bar Charts of Counts

Distributions:

• The distribution of a variable (a column) describes the frequency of its different values
• The group method counts the number of rows for each value in a column

Bar charts can display the distribution of categorical values

• Proportion of how many US residents are male or female
• Count of how many top movies were released by each studio

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(Demo)

Categorical Distributions

bar chart: barh

Displays a categorical distribution

(But when the values of the variable have a rank ordering, or fixed sizes relative to each other, more care might be needed.)