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1.3 Data: Sources and Types 55

TABLE 1.1 Selected Observations on Test Scores and Other Variables for California School
Districts in 1999

Observation (District) District Average Student–Teacher Expenditure per Percentage of Students
Ratio Pupil ($) Learning English
Number Test Score (fifth grade) 17.89 $6385 0.0%
21.52 5099 4.6
1 690.8 18.70 5502 30.0
17.36 7102 0.0
2 661.2 18.67 5236 13.9
. . .
3 643.6 . . .
. . .
4 647.7 21.89 4403 24.3
20.20 4776 3.0
5 640.8 19.04 5993 5.0
..
..
..

418 645.0

419 672.2

420 655.8

Note: The California test score data set is described in Appendix 4.1.

Achievement Test). The average student–teacher ratio in that district is 17.89; that
is, the number of students in district #1 divided by the number of classroom teachers
in district #1 is 17.89. Average expenditure per pupil in district #1 is $6385. The
percentage of students in that district still learning English—that is, the percentage
of students for whom English is a second language and who are not yet proficient
in English—is 0%.

The remaining rows present data for other districts. The order of the rows is
arbitrary, and the number of the district, which is called the observation number,
is an arbitrarily assigned number that organizes the data. As you can see in the
table, all the variables listed vary considerably.

With cross-sectional data, we can learn about relationships among variables
by studying differences across people, firms, or other economic entities during a
single time period.

Time Series Data

Time series data are data for a single entity (person, firm, country) collected at
multiple time periods. Our data set on the growth rate of GDP and the term
spread in the United States is an example of a time series data set. The data set