Births and Holidays

Data Computing

Computing project

The number of daily births in the US varies over the year and from day to day. What’s surprising to many people is that the variation from one day to the next can be huge: some days have only about 80% as many births as others. Why?

The data table Birthdays in the mosaicData package gives the number of births recorded on each day of the year in each state from 1969 to 1988. (It would be nice to have more recent data, but I don’t have them at hand.) For this activity, we’ll work with data aggregated across the states.

  1. Create a new data table, DailyBirths, that adds up all the births for each day across all the states. Plot out daily births vs date.

The date variable in Birthdays prints out in the conventional, human-readable way. But it is actually in a format (called POSIX date format) that automatically respects the order of time. The lubridate package contains helpful functions that will extract various information about any date. Here are some you might find useful:

Using these lubridate functions, you can easily look at the data in more detail.

  1. To examine seasonality in birth rates, look at the number of births aggregated over all the years by
    1. each week
    2. each month
    3. each Julian day

  2. To examine patterns within the week, look at the number of births by day of the week.

  3. Pick a two-year span of the Birthdays that falls in the 1980s, say, 1980/1981. Extract out the data just in this interval, calling it MyTwoYears. (Hint: filter(), year()). Plot out the births in this two-year span day by day. Color each date according to its day of the week. Explain the pattern that you see.

Births and holidays

  1. A few days each year don’t follow the pattern in (4). We’re going to examine the hypothesis that these are holidays. You can find a data set listing US federal holidays at http://tiny.cc/dcf/US-Holidays.csv. Read it in as follows:1 The point of the lubridate::dmy() function is to convert the character-string date stored in the CSV to a POSIX date-number.
Holidays <- read.csv("http://tiny.cc/dcf/US-Holidays.csv") %>%
  mutate(date = as.POSIXct(lubridate::dmy(date)))
  1. Add a couple of layers to your plot from (4).
    1. Draw a vertical bar at each date which is a holiday. You’ll use the geom_vline() glyph. You can give a data = argument to geom_vline() to tell it to plot out the information from Holidays rather than MyTwoYears.2 Unfortunately, due to what I believe is a bug in geom_vline(), you will have to set the x position of the bars by as.numeric(date) rather than just date().
    2. Add a text label to each of the vertical bars to identify which holiday it is. Use the geom_text() glyph.3 Hints: You’ll have to make up a y-coordinate for each label. You can set the orientation of each label with the angle aesthetic.

  1. The plot in (6) is too busy. Let’s explore some other ways to display the data to make it clearer to the view that holidays tend to be low-birth days.

Add a variable to MyTwoYears called is_holiday. It should be TRUE when the day is a holiday, and FALSE otherwise. One way to do this is with the transformation verb %in%, for instance,

is_holiday = date %in% Holidays$date

Make a new plot where you map is_holiday to the shape or the size aesthetics, or perhaps for faceting. Or perhaps change the way color is used in the graph to show weekends and holidays separately from non-holiday weekdays. Make the graph as simple as you can until you get one that clearly tells the story. You may want to simplify by eliminating any components of the graph (e.g. holiday labels? dots? lines? vertical lines?) that aren’t essential to telling the story.