Carnes
UNDERSTANDING INDEPENDENT & DEPENDENT VARIABLES
CP Biology
Lesson Goals/Objectives:
Students will understand the relationship between an independent variable and a dependent variable
by graphing observable data. Students will be able to use collected data to make bar graphs and line
graphs. Students will be able to interpret graphs and discuss the relationship between independent
and dependent variables in an experiment.
Background:
Whenever possible, a hypothesis should be tested by an experiment in which only one variable is
changed at a time:
 All other variables should be kept unchanged, or controlled.
 This type of experiment is called a controlled experiment:
□ Manipulated variable (independent)
 This is the variable that the scientist manipulates, or changes in some
way.
 Always graphed on X-axis!
□ Responding variable (dependent)
 This variable responds (changes in response) to the change made to the
independent variable.
 Always graphed on Y-axis!
Breaking this down further, we summarize independent and dependent variables in this way:



Independent Variables are changes that occur in an experiment that are directly caused
by the experimenter (you.)
Dependent Variables are changes that occur due to independent variables.
A Controlled Variable is anything else that could influence the dependent variables.
Controlled variables must be carefully monitored and kept equal in your experiments - otherwise they
could mess up your experiment by making your results false or unreliable.
Reminder: A hypothesis is a possible general explanation for a set of observations or an answer to a
scientific question:
 In science, a hypothesis is useful only if it can be tested!
 Hypotheses must be falsifiable!
Hypotheses may arise from prior knowledge, logical inferences, or educated guesses:
 Hypotheses are tested via controlled experiments (an experiment in which only one
variable is tested at any time).
 ALL other variables should be kept unchanged (controlled)
The easiest way to state your hypothesis in a way that will help you to determine your
independent/dependent variables is to put it in the “if/then” format:
“If I ________________________, then _____________________________.”
“If I (do this), then (this will happen).”
Carnes
“If I (independent variable), then (dependent variable).”
Procedure:
Now that we have reviewed the meaning of the terms relevant to this lesson, let’s see if we can begin
to understand the relationship between an independent variable and a dependent variable. Read the
following hypothesis and try to pick out the variables:
 Hypothesis: If I increase the amount of sunlight per day that a plant receives, then the
plant will grow faster.
□ So what is the independent variable?
 Answer: amount of sunlight
□ And what is the dependent variable?
 Answer: growth rate of plan
Now, can you explain the relationship between these two variables? Sometimes, this is more easily
understood by using a visual representation of the relationship, such as a chart or graph. Let’s use
the following “data” to make a graph that expresses the relationship between the variables in an
experiment.
Here is your “data”:
Plant # 1
Plant # 2
Plant # 3
Plant # 4
Plant # 5
Amount of
Sunlight/Day
2 hours
8 hours
4 hours
12 hours
6 hours
Average Growth
Rate/week
2 inches
6 inches
3 inches
1 inches
8 inches
Just by looking at the date in the chart above, can you verbally explain the relationship between the
independent variable (amount of light) and the dependent variable (growth rate of plants)? Let’s see
if this becomes clearer as we put the data into a chart or graph. Follow the instructions below for
creating your graph using Excel®:
1. Open up a new Excel® spreadsheet on your computer.
2. As shown in the chart above, place the data into the spreadsheet (see image below):
 Note that the independent variable (amount of sunlight) is placed in the first column while
the dependent variable (growth of plants) is placed in the second column. The headers at
the top of each column are not necessary, but they do help identify the variables.
Carnes
3. Now that we have our data in our spreadsheet, let’s create a bar graph. Bar graphs are a very
common type of graph best suited for a qualitative independent variable.
a. With one independent variable and one dependent variable, a simple bar graph will work
best.
b. Highlight all data in your chart.
c. Start the chart wizard from the toolbar:
d. If the chart wizard is not available on the toolbar, you can also choose Insert > Chart...
e. Choose the Column Chart type and the Chart sub-type in the upper left corner (basic
bar graph). This chart type creates a vertical bar graph, which Excel® refers to as a
Column chart. Click Next when you are done.
Carnes
f. Confirm that your Data Series are in Columns in your spreadsheet. Your Data range
should reflect your selection of the independent and dependent data (plus possibly your
column headers) in absolute cell references. The preview should show a pretty good
representation of what your chart will look like. Click Next when you are done.

To “grab” your data, click on the
icon. Highlight all text typed, then click the
icon again. This will put your data into your graph – see image below:

Click Next.
g. Enter your Titling. Remember that the independent variable is on the X-axis and the
dependent variable is on the Y-axis!
h. Click Next when you are done.
Carnes
i.
Your final graph should look something like the one below when you are finished. Note that
when the graph is selected, your independent and dependent variables are highlighted in
purple and blue boxes, respectively.
 Select “As object in” sheet 1 to view your graph in the current spreadsheet you
have already opened.
 Select finish.
Carnes
4. Now, using the graph you have created like the one above, can you explain the relationship
between the independent/dependent variable?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
 Students should respond that according to the data, the more daily sunlight a plant
receives, the faster it will grow up to a certain point – but that too much exposure to sun will
cause the plant growth to be stunted.
Now let’s use different data to create a different type of graph. A scatter plot graph/line graph is
often used to express the relationship between 2 variables (such as an independent/dependent
variable relationship). Use the following data and instructions to create your line graph:
Amount of Study Time
Test Grade
1 hour
2 hours
3 hours
50
60
75
Carnes
4 hours
5 hours
90
100
5. Open up a new Excel® spreadsheet and enter your data into two columns.
6. Highlight your entered data and select the chart wizard
.
7. Choose the chart type (XY Scatter) and chart sub-type Scatter. Click Next.
a. View a preview of your scatter chart and click Next again.
b. Enter your chart titles. Remember that the independent variable is on the X-axis and the
dependent variable is on the Y-axis!
c. Click on the Legend tab and click of the “show legend” box – you do not need a legend for
a graph with only one independent/dependent variable.
d. Click Next.
e. Select “As Object in:” sheet 1. Then click Finish. Your graph should look very similar to
the example below:
Carnes
8. Now, using the line graph you have created above, can you explain the relationship between the
independent/dependent variable? In your response, explain how the use of graphs can make
identifying these types of relationships easier!
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
 Students should respond that according to the data, the more daily hours they study, the
higher their test grade is likely to be!
CONGRATULATIONS!
You now understand the
relationship between an
independent variable and
a dependent variable! You
have also learned how to
express these
relationships using charts
& graphs! Now let’s
practice some more!
Affordances:
Using graphs to express the relationship between independent and dependent variables is valuable in
helping students to understand the use of variables in scientific experimentation. Often times,
students will find it easier to identify independent/dependent variables when looking at a “visual”
representation of data as opposed to text. While many types of graphs can be hand-drawn, allowing
Carnes
students to create graphs/charts in Excel saves valuable class time while integrating technology with
instruction.