Cases, Variables, and Values

A data table is comprises cases and variables.

Each variable comprises values (or levels).

There is no hard distinction between a variable and a value. What’s a variable in one situation may be a value in another, and vice versa.

A data table

Students

Two formats

• Data in Key/Value format are narrow
• The corresponding wide format has
  • separate variables for each level in Key
  • sets the values for those variables from the info in Value

Narrow

Wide

Gather — from Wide to Narrow

Syntax:

WideInput %>% 
  gather(key_name, value_name, ...)

The ... are the variables to be gathered together, e.g.

StudentsNarrow <- Students %>% gather(key, value, x, y)
StudentsNarrow

Cases in Narrow data

Aside from Key and Value, all the other variables identify the case.

The gathering makes multiple rows for each row in the wide form. The variables not used for narrowing are copied into the new multiple cases.

Why?

Some operations are easy in wide format, but hard in narrow and vice versa

Excerpt from BabyNames

Questions:

1. How many babies of each name and sex?
2. For each name, is it primarily given to girls or boys? Which names are gender neutral?

In narrow format

1. How many babies of each name and sex?

BabyTotals <-
  BabyNames %>%
  group_by(name, sex) %>%
  summarise(total = sum(count))

Easy!

In Wide format

2. For each name, is it primarily given to girls or boys? Which names are gender neutral?

• sex is the key variable
• total is the value variable

BabyTotalsWide <- BabyTotals %>% 
  spread(sex, total)
BabyTotalsWide

With sexes side by side …

BabyTotalsWide %>% 
  mutate(gender_specificity = pmax(F/(M+F), M/(M+F)))

Homework

• DC Ch 11 Exercises: 11.1, 11.3, 11.4, 11.5
  
• DC chapter 12 & 15 reading quiz on Canvas
  
• Start thinking about your Final Project

Activity: Stocks & Dividends (DC p. 170)

Note: SKIP “Dividends” section beginning on p. 173

Assignment is worth a total of 10 points.

• [2 points] Turn in HTML with embedded .Rmd file (e.g. “DataComputing simple” template)
• [1 points] Use ggplot to chart stock price over time for a few stocks (stocks chosen may vary)
  • Use Solstice to share your chart on the front screen
• [2 points] Proper join of Prices table and Actions table
  • Draw the intended result on your white board before programming
• [2 points] Calculate Stock Profits and show result in “Wide” format of SalesDifference table
  • Draw the intended result on your white board before programming
• [2 points] Proper join of Prices table and Reference table
  • Draw the intended result on your white board before programming
• [1 points] Use ggplot to chart index over time for chosen stocks (stocks chosen may vary)