Basic Graphing
Basic Skills
Graphing Variables
•
•
The textbook discusses dependent and
independent variables in the section on
scientific method. Many times it is valuable
to plot data of these variables on a graph to
better visualize any potential relationship
between them.
In math class you usually practice plotting
paired data (x, y) where the two variables
don’t really have any meaning. In science
class, on the other hand, the variables are
real things that have meaning. For instance,
you might make a graph of distance versus
time, or volume versus pressure. It is
traditional to plot the independent variable
on the “x” axis, and the dependent variable
on the “y” axis.
Boyle’s Law: Volume vs. Pressure
2
Graphical Relationships
• The sample graphs show
examples of both linear
(straight line) relationships, and
non-linear relationships (more
complex mathematical curves).
• The graphs also show examples
of “x” and “y” being positively
correlated (a direct relationship
meaning as “x” increases in
value, so does “y”), and
negatively correlated (an
indirect or inverse relationship
meaning as “x” increases in
value, “y” decreases).
3
Rules for Making a Graph
• Title the graph. The title should clearly
state the purpose of the graph and may
include the independent and dependent
variables.
• Plot the independent variable on the
horizontal “x” axis and the dependent
variable on the vertical “y” axis.
• Label each axis with the name of the
variable and the unit. For example in the
above graph, the “x” axis is measuring
“Pressure” measured in units of
“atmospheres”.
Boyle’s Law: Volume vs. Pressure
4
Rules for Making a Graph
• Choose your scale carefully. Generally
start at zero. Make each “tick mark” the
same measure. Look at the range of the
values of the data you are plotting to
help determine the maximum value on
the scale. Make your graph as large as
possible by spreading out the data on
each axis.
• Notice you do not need to have the same
scale on each axis like you usually do in
math class. In the above graph, each
“tick mark” on the “y” axis is 2 liters,
whereas each “tick mark” on the “x” axis
is 1 atmosphere.
Boyle’s Law: Volume vs. Pressure
5
Rules for Making a Graph
•
•
If the data points appear to lie roughly in a straight line,
draw the “best fit” straight line you can with a ruler and
sharp pencil. Have the line go through as many points as
possible with approximately the same number of points
above the line as below. Generally do not “connect the
dots”. If the points do not form a straight line, draw the
best smooth curve possible. See examples to the right.
Sometimes, but not always, there is a good reason for the
“best fit” line to go through the origin (0,0). Think about
your experiment and decide if the data would logically
include a (0,0) point. For example, if plotting speed vs.
time and an object that is at rest when you start a timer,
then your graph would go through the origin (the speed is
zero when time is zero). If the object is already in motion
when you start the timer, your graph will not go through
the origin.
6
Pie Charts
7
Histograms
8
Correlation Between Variables
• If two variables are correlated, then there is some kind of relationship or
association between them. Possibly as one variable increases, so does
the other – a positive correlation. You could also have negative
correlations.
• It is important to note that even if there is some kind of correlation
between the two variables that does not necessarily prove there is a
cause-and-effect relationship. Your background knowledge and
judgment must help decide that.
9