Graphing in the Biology Classroom Making sense of Evidence

Displaying Data A Graph is a diagram that shows the relationship of one variable to another. Why do we use Graphs? Graphs help visualize numerical data There are several different types of graphs Bar Graphs Line Graphs Pie Graphs

Why do we need to be able to graph data? Graphing of data, that is the physical representation of the data, is a tool used by scientists, researchers, teachers, and students to make visual sense of a “sea of numbers” Graphing shows trends or implies similarities or differences in data that was collected during the course of an investigation or experiment. Without graphs, many people have a very difficult time understanding exactly what the data has shown. However, graphs can sometimes be misleading, especially when they are constructed improperly. Many times the wrong construction of a graph detracts from the acceptance of an individual’s hypothesis.

“But I don’t like to graph! Can’t I show my data another way??” Sure! Graphing is nothing more than a tool used to help people understand. A hammer is a tool used to pound nails, a wrench is a tool used to turn a bolt, a dump truck is a tool used to carry heavy loads. In the same manner, a graph is a tool that we use to perform a task or more accurately show something. So, if you don’t like graphs, try to pound a nail without a hammer or carry a bunch of rocks one by one to dump into your driveway.

“OK, so I need to be able to draw a graph. What if I mess it up?” Wait a minute….. I never said we had to be perfect the first time! Graphing is an accomplished skill. The more you do it, the better you become! You are going to practice and develop your graphing skills. Start by being prepared Make sure you have the tools your need! Using a sharp pencil is the best thing. Using a pen will lead only to intense frustration for you and excessive copying of replacement pages for your teacher. Also, if your pencil is your sword, your good quality eraser is your shield. Have one handy to help you fix any mistakes. You will do best to start with scratch paper: pre-draw or rough sketch the graph before you do it for the final time on your answer paper.

Know what kind of graph you need Traditionally, line graphs are used in scientific situations, but occasionally we also use pie charts, bar graphs, and sometimes even a histogram. So how do you decide which type of graph to use?

Bar Graph A Bar Graph is used to show a comparison of multiple objects A Bar Graph should be used for “one-time” measurements.

Line Graph A Line Graph best represents “change over time” data. A Line Graph is used to show the representation between variables A Line Graph is used when you have multiple measurements

Pie Graph A Pie Graph is used to compare the parts of a whole. A Pie Graph can clearly display percentages A Pie Graph is never used unless the data adds up to “one” or “one hundred”

Terms for Graphing A graph contains five major parts: a title the independent variable the dependent variable the scales for each variable recorded on the axis and a key to the marks, lines, or different types of data

Title Your Graph BE DESCRIPTIVE The name of the graph is important Nothing “cute” here. It must tell what the graph is showing. For instance…”Plant growth in centimeters per day for Plant A” is a good title. “How big is my plant?” is not a good title.

Determine your X and Y Axes The Independent Variable: is the variable that can be controlled by the experimenter. It usually includes time (dates, minutes, hours), depth (feet, meters), temperature (degrees Celsius). It is placed on the X axis (horizontal axis) because it is the MOST CONSISTENT variable. The Dependent Variable: is the variable that is directly affected by the independent variable. It is the result of what happens because of the independent variable. It is what you are measuring. Example: How many bubbles are produced in an enzyme reaction?… what is the volume of gas generated by a plant in one hour?, etc… The bubbles are dependent on the rate of reaction, the volume of gas is dependent on the intensity of light. This variable is placed on the Y-axis or vertical axis.

The Scales for each Variable In constructing a graph you need to know where to plot the points represented by the data that you collected. In order to do you must engineer your scale so that it will include all the data points. Find the highest value for your data set and then count the available spaces on the appropriate axis of your graph. Think about this one… if you have 30 spaces on your graph and your data goes to 28, you can safely assume that each “box” has a value of “1” for that data set. However, if you only have 15 spaces, it is safe to assume that each box has a value of “2”. USE THE SPACE EFFECTIVELY. MAINTAIN THE SCALE If you determine that one space will equal two points, you must maintain that value. You can’t change it to 4 points further up the axis or with another sample.

Look at your graph Do the lines look correct given the data? Does the data do REALLY weird things (extreme highs and extreme lows very quickly, lines stay at one value for a long duration, etc…) If so, perhaps it is time to do another rough draft of the graph and see if you should change the scale or if you possibly made a mistake Does your graph make sense. Do you need a key to clarify abbreviations, colors, data points, comparisons.

Now - Practice