Created by Virginia Schutte
Pre-class assignment 1: graphing
SECTION 1: Types of graphs
There are several common types of graphs that can be used to display information visually. We
will cover: 1) pie charts, 2) bar graphs, 3) scatter plots, and 4) line graphs.
1) Pie chart: This type of graph is used to display parts of a whole (for example, of all the
organisms counted in a stream, how many are plants and how many are animals?). This graph is
best used to convey information about something’s composition. However, the results are
relative in this type of chart (for example, the number of plants in the stream is converted to a
percentage of all the organisms that were counted- the number displayed is relative to that total).
Variables
A variable is something that is measured, controlled, or manipulated in research. For example,
in an experiment where I test whether drinking milk affects bone density, both bone density and
amount of milk consumed are variables. The variable that is suspected of controlling the results
we measure is called the independent variable and the variable that we suspect is controlled by
the independent variable is called the dependent variable, because its results are affected by
(dependent on) the independent variable. In the example above, milk consumed is the
independent variable and bone density is the dependent variable. In bar, scatter, and line graphs,
the independent variable is plotted on the x-axis and the dependent variable is plotted on the yaxis.
Pre-class assignment 1: graphing
2) Bar graph: This type of graph shows the values of different measurements taken (for
example, how many plants and how many animals were counted in the stream?). This graph is
best used to convey information about quantities (how much there is of something; for example,
4 cups of water is a quantity) measured in association with categorical variables. Categorical
variables are groupings created to classify the items measured. Categorical variables include
variables like foods, colors, and gender and can include variables like height, weight, and speed
if they are classified into different groups (classifying speeds into groups like slow and fast or
45-55 mph would be appropriate for a bar graph). This graph displays absolute results- the
numbers collected are not made relative to anything (for example, this graph will display the
actual number of plants and animals that were counted in the stream).
3) Scatter plot: This type of graph shows the values of measurements that were taken (the
same type of information as a bar graph), but is best used to relate those measurements to a
continuous variable (for example, how does location influence the number of plants in the
stream?). Continuous variables are the opposite of categorical variables- they are measured over
a continual scale and are not grouped into different classifications. Continuous variables include
variables like time, height, and size and do NOT include variables like colors, college attended,
or age unless they are converted to continuous variables (for example, 23.4 and 65.7 years old
instead of young and old).
Pre-class assignment 1: graphing
4) Line graph: This type of graph also involves a continuous variable and can be thought of as
a scatter plot with a line drawn through the values displayed on the chart. This graph is best used
to convey information about changes to a dependent variable over independent variable values
that are related to each other (for example, how has the number of plants in the stream changed
over the last year? The independent variable values (months sampled) are related to each other
because the same location was sampled each time). This type of graph is frequently used when
conveying information over time.
Pre-class assignment 1: graphing
SECTION 2: Making graphs
Graphs can be made by hand but there are many computer programs that can be used to make
graphs more quickly. One of the most common and perhaps the easiest to use is Microsoft
Excel. We will cover how to make each of the graphs introduced in section 1 in versions of
Microsoft Excel released in 2007 or later (available in the computer labs on campus).
Components of a good graph
All graphs must have a title (NOTE: this is often replaced by a figure legend, a brief description
of the graph printed below the figure, in scientific publications), variable labels for the things
measured, and (if appropriate) axis labels (with units like g [grams] in your axis labels if
appropriate). For example:
Pre-class assignment 1: graphing
How to make graphs in Excel 2007
NOTE: The instructions below should enable you to produce graphs identical to those given
above as examples in Section 1. Try following these instructions and use the example graphs
above to verify that you have followed the instructions correctly.
1) Pie charts:
1. Type in the following data so you have a column with what you measured (in this
case, organisms) and list the specific names of the variables you measured underneath that. Then
make a second column with the title of the graph, then list the proportions of organisms that you
calculated underneath that:
Organisms: plants- 65, animals- 35
2. Select the cells that contain the information you just entered by clicking and dragging
until the 6 cells you modified are highlighted. Then click on the “insert” tab (2nd from the left at
the top of your screen), then click on “Pie” in the “Charts” section at the top of your screen, and
select the simplest type of pie chart, the top left icon in the “2-D Pie” section at the top of the
drop down menu. Your graph should appear in your excel worksheet once you have selected this
icon.
Pre-class assignment 1: graphing
3. To add a title to your graph (if one is not automatically added), select the chart, then
click on the “layout” tab (2nd from the right), then click on “chart title”, and “above chart”.
4. To add a legend to your graph (if one is not automatically added), while the chart is
selected click on the “layout” tab, then click on “legend” and choose where you would like the
legend to be displayed.
Pre-class assignment 1: graphing
5. You can reformat many aspects of the graph according to your preferences by clicking
on various components of the graph (like the title), then clicking on the “Layout” tab that is only
present when some aspect of your graph is selected (2nd from the right), then clicking on “Format
Selection” in the “Current Selection” section at the top of your screen.
2) Bar graphs:
1. Enter your data in the same way as was described in step 1 above, but this time use the
following abundance data:
Type of organism: plants- 11, animals- 6
2. Select the cells with relevant information and click “insert” as you did for step 2
above, but click on “Column” in the “Charts” section and select the simplest type of bar graph,
the top left icon in the “2-D Column” section at the top of the drop down menu. Your graph
should appear in your excel worksheet once you have selected this icon. NOTE: although Excel
distinguishes between column and bar graphs, scientists commonly call both types bar graphs.
Pre-class assignment 1: graphing
3. Make sure your graph has a title by following the instructions from step 3 above.
4. To add a label for the x-axis onto your graph, while the chart is selected click on the
“layout” tab, then click on “axis titles”. Select “primary horizontal axis” and click on “title
below axis”. To add a label for the y-axis onto your graph, select “primary vertical axis” and
click on “rotated title”.
5. You can reformat many aspects of the graph according to your preferences. For
example, a legend here is not necessary because you only have one variable on your y-axis (thus
you do not need to distinguish between that variable and anything else). To delete the legend
automatically created for you, simply click on it and press “Delete” on your keyboard.
Pre-class assignment 1: graphing
3) Scatter plots:
1. Enter your data in the same way as was described in step 1 above, but this time use the
following location and abundance data:
Location: mile - 1.5, plants- 8; mile 3, plants 6; 3.5, 9; 5, 8; 6, 7; 6.25, 8; 6.5, 6; 7, 10
2. Select the cells with relevant information and click “insert” as you did for step 2
above, but click on “Scatter” in the “Charts” section and select the simplest type of scatter plot,
the top left icon in the drop down menu. Your graph should appear in your excel worksheet once
you have selected this icon.
3. Make sure your graph has a title by following the instructions from step 3 above.
4. Make sure your graph has axis labels by following the instructions from step 4 above.
5. You can reformat many aspects of the graph according to your preferences.
Pre-class assignment 1: graphing
4) Line graphs:
1. Type in the following data so you have a column with when you measured (in this
case, month) and list the specific names of the timepoints you measured underneath that. Then
make a second column with the title of the graph, then list the abundances of organisms
underneath that:
Plants: February- 11, May-15, August- 22, November- 13
2. Select the cells with relevant information and click “insert” as you did for step 2
above, but click on “Line” in the “Charts” section and select the basic type of line graph, the
bottom left icon in the “2-D Line” section at the top of the drop down menu. Your graph should
appear in your excel worksheet once you have selected this icon.
3. Make sure your graph has a title by following the instructions from step 3 above.
4. Make sure your graph has axis labels by following the instructions from step 4 above.
5. You can reformat many aspects of the graph according to your preferences.
Pre-class assignment 1: graphing
SECTION 3: Reading graphs
There are 4 basic steps to reading and interpreting a graph. We will walk through these 4 steps
using the line graph shown below as an example.
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Step 1: What type of graph is it?
It is a line graph, so changes in data are being related to a continuous variable.
Step 2: What are the axes (units and scale)?
x-axis: Time (in months; every 3 months from Feb - Nov)
y-axis: Plant abundance (number of plants counted in the Gotham City stream)
This tells us that the abundance of plants in the Gotham City stream was
counted every 3 months from February to November.
Step 3: Is there a pattern or trend in the data?
The data increase from Feb - May, increase from May - Aug, and decrease from Aug Nov.
This means that there is no constantly upward or downward trend in plant
abundance through time. Plant abundance peaked in August and was for
the most part consistent otherwise.
Step 4: What can you conclude from the pattern you described?
The plants examined are most abundant in early fall and during the rest of the year are
approximately half of their peak abundance.
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Source: FIRST IV (Faculty Institutes for Reforming Science Teaching) program
Pre-class assignment 1: graphing
SECTION 4: Help and glossary
I want information on:
- deciding which type of graph is most appropriate for my data. We suggest:
http://www.mindtools.com/pages/article/Charts_and_Diagrams.htm for a review of graph
components and more detailed guidance on which graphs are suited for which types of data.
NOTE: while this site contains good graphing examples, their graphs do not always contain the
necessary components required of a good graph (see Section 2 above).
- making a graph in Excel: Excel’s “help” button is shaped like a question mark in a
circle and is in the upper right corner of your screen underneath the minimize button.
- practicing graphing beyond this exercise. We recommend:
http://42explore.com/graphs.htm for a list of websites with more graphing information and
tutorials.
- understanding graphs. We suggest:
http://www.galeschools.com/research_tools/src/interpret_graph.htm for a basic guide on graph
interpretation.
- using Excel. We suggest: http://www.ncsu.edu/labwrite/res/gt/gt-cell-home.html and
http://staff.tuhsd.k12.az.us/gfoster/standard/excelgra.htm which, even though they are written for
an earlier version of Excel, have basic usage information.
- why I should care about graphing. This exercise is intended to be a basic introduction
to or review of graphing principles, but there are many (and more complex) types of graphs that
can be applied across disciplines and may be important to any career. Here are 2 websites with
numerous examples of how graphs can be used in non-science situations:
http://www.visualcomplexity.com/vc/, http://www.aisee.com/gallery/
*** If you have more graphing questions, please ask a TA or the professor- you WILL be
_________expected to interpret and create graphs throughout this semester ***_________
bar graph: A graph with bars whose lengths are proportional to the quantities of a measured
variable
categorical variable: A variable that is placed into discrete categories (ex: breed of dog)
continuous variable: A variable that can take any possible value within the range limits of the
variable (for example, price)
legend: List of variables so the reader can differentiate between 2 or more variables displayed in
a graph
line graph: A graph in which a line is used to connect the quantities of a measured variable
pie chart: A circular graph that is divided proportionally to represent the relative quantities of a
measured variable
relative results: Data that have been normalized, or made relative to each other (for example, 2
is 50% of 4)
scatter plot: A graph where quantities of a measured variable are plotted on a coordinate plane
made by more than 1 axis
variable: A quantity that can take any of a set of values (for example, in an experiment where I
test whether drinking milk affects bone density, both bone density and amount of milk
consumed are variables)
x-axis: The horizontal axis of a two-dimensional graph
y-axis: The vertical axis of a two-dimensional graph