All of the data verbs we have used up to now take a single data table as an input.

Sometimes you need to combine data between different sources.

• Perhaps they come from different experiments or institutions.
• Perhaps they are completely different types of data, e.g. medical treatment records and demographic or education data.
  • Example: MedicareCharges, MedicareProviders, DirectRecoveryGroups.
  • Example: the registrar's database

Slapping one data table onto another row-wise is sometimes an appropriate activity, but it is … not a join!

Example of using row-wise concatenation:

• Each day's new earthquakes are available through a geophysics site. You're collecting all the events over a long period of time.

You're TA just entered the students grades for assignment 7 into a spreadsheet, sorted by student name.

You have your master spreadsheet with assignments 1 through 6.

Concatenate the assignment-7 grades as a new variable for your master spreadsheet.

** Watch out!** This is a brittle process.

• Suppose your TA's sheet has sorted the students by last name and yours has them sorted by first name.
• Suppose you delete students who drop the course, but the TA doesn't have this information.

1. Match the students in one table to those in the other.
2. Gently lift the Assignment-7 data for each student from the TA's table into the appropriate cell in your table.

This is one example of a join. There are other kinds of join for situations where there might be multiple matches between cases in the two tables.

Continuation of the example here

Here are three data tables relating student grades in courses at Macalester in 2005 (?)

grades <- read.csv("http://tiny.cc/mosaic/grades.csv",
                   stringsAsFactors = FALSE)
courses <- read.csv("http://tiny.cc/mosaic/courses.csv",
                    stringsAsFactors = FALSE)
grade_to_number <- read.csv("http://tiny.cc/mosaic/grade-to-number.csv",
                            stringsAsFactors = FALSE)

1. How many students in each department?
2. What fraction of grades are below B+ in each department?
3. What's the grade-point average (GPA) for each student?
4. What's the 95% confidence interval on the GPA for each student?
5. Grade-point average for each department or instructor
6. (Statistically more sophisticated) To what extent does the grade reflect the student or the department or instructor?

Birds at the Ordway Nature Preserve

devtools::install_github("DataComputing/DataComputing")
require(DataComputing)
View(OrdwayBirds)
help(OrdwayBirds)

As a group, let's correct the species names using this data table.

Count how many birds there are of each species in OrdwayBirds using the corrected species names.