Visualizing Environmental Science

Graphing Appendix 467

Graphing Nuts and Bolts

LEARNING OBJECTIVES

1. Define data and variable.


2. Distinguish between independent and dependent
   variables.


3. Describe the following components of a graph:
   x-axis, y-axis, title, caption, legend, future projection,
   and measurement unit.

I

n science, data simply refers to factual infor-
mation. Note that because one piece of
factual information is called a “datum,” even
though it may sound funny, scientists are
grammatically correct when they make statements such
as “These temperature data are important because ...” or
“This temperature datum is important because ...”
A variable is a factor whose values can change. Many
graphs illustrate the relationship between values of a
dependent variable and values of an independent variable.
As the names imply, the values of a dependent variable
are assumed to depend on the values of an independent
variable. To investigate complex interactions and test a
series of competing hypotheses, scientists often examine
how the values of one independent variable may affect
the values of one or more other dependent variables.
For example, suppose an ecologist hypothesizes that
the complexity of plant communities directly affects the
number of bird species present in a given area. This ecolo-
gist could test this hypothesis by recording the number of
bird species in different areas and quantifying the com-
plexity of the vegetation in each of these areas. These data
could then be presented in a graph like this (Figure A.2):

By convention, the independent variable is always pre-

sented on the horizontal or x-axis and the dependent vari-

able is always presented on the vertical or y-axis. Thus in

this case, the ecologist assumes that the number of bird spe-

cies depends on the structural complexity of the vegetation.

In addition to the x- and y-axes, it is important to rec-

ognize and understand the other components that are

typically used to construct scientific graphs. As illustrated

in the figure below (Figure A.3), most graphs start with a

title, which succinctly describes the information they pres-

ent. There is often a caption beneath this title that states

the major implication of the data presented in the graph.

Graphs that portray more than one series of data such as

this one may use a legend that distinguishes and explains

each of these data series. Reading a graph’s title, caption,

and legend before examining its actual data will often help

you better understand and interpret its purpose and results.

Structural complexity of vegetation

Number of bird species

0 0.4 0.8 1.2 1.6 2.0

10

20

30

40

Independent variable

x-axis

Dependent variable

y-axis

After M.L. Cody and J.M. Diamond, eds., Ecology and Evolution of Communities

.

Harvard University, Cambridge (1975).

Figure A.2

1960–1965 2011

Brazil China Egypt India Mexico Nigeria

8.0

6.0

4.0

2.0

Total fertility rate

Legend

Caption

Title

Based on data from Population Reference Bureau.

ˆ}ÕÀiÊ
°ÎÊUÊiÀ̈ˆÌÞÊV…>˜}iÃʈ˜ÊÃiiVÌi`Ê

developing countries

Since the 1960s, fertility levels have dropped

dramatically in many developing countries.

Some graphs illustrate how the relationship between

two variables may change in the future. These future

projections are usually represented by a dashed line

(Figure A.4, next page). In this case, the graph projects

the rate of future human population growth as a func-

tion of three different estimated fertility rates. Also note