Changes over time

One of the most important functions of Official Statistics is to monitor changes of societies over time. Many demographic, social and economic measurements are recorded monthly or annually and systematic changes to these measurements are important to detect and report.

The simplest form of time series consists of a single measurement that is recorded at regular intervals (usually yearly, monthly or daily). A scatterplot of the values against time is called a time series plot or time series chart. By convention, time is always plotted on the horizontal axis and successive crosses are usually joined by lines.

time series plot


Defence Investment in USA

The table below describes the investment in defence in the USA between 1947 and 2006. (Because 1947 dollars were worth less than 2006 dollars, all values have been adjusted for inflation and are reported in 2000 dollars', allowing more meaningful comparisons between the years)

Real National Defense Gross Investment (2000 $billion)
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
4.9
9.4
13.8
10.8
38.5
55.7
62.4
50.8
41.1
39.3
37.3
39.7
47.3
42.7
47.3
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
48.9
42.3
38.4
33.5
38.7
43.7
38.3
34.0
30.9
19.7
14.7
16.6
20.2
22.5
24.5
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
25.0
24.8
28.4
30.4
33.5
37.7
44.2
50.5
60.7
69.6
76.0
70.3
70.7
72.9
68.7
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
66.1
57.8
53.8
50.9
51.3
45.1
45.6
47.6
48.8
50.8
56.7
61.7
68.0
71.7
76.6

Although it is possible to see some patterns in the table above, the trends are clearer in a time series plot.

Click on crosses to read off exact expenditures. It is interesting to try to associate the peaks and dips in this time series plot with government administrations and international events! A manufacturer in the defence sector might also hope to predict future government expenditure from the trend in recent years.

Pigs in Thailand

The time series plot below shows the numbers of pigs in Thailand each year between 1960 and 2006.

Click on crosses to read off the number of pigs in any year.

Regional population growth in New Zealand

Our final four time series describe the population growth rates in the four regions of New Zealand with the largest urban populations between 1996 and 2006. Predictions of population growth are important to governments — the infrastructure of education and health services must be expanded to cope with increases — and to retail businesses who will want to site outlets in areas of growth.

Observe that: