6 1.5 Graphing Introduction
Biomechanics researchers collect a lot of data (numbers) to understand human movement. These numbers have to be interpreted and presented to the readers. For example, if researchers want to see if carbon insoles help you jump higher than regular insoles they would have to collect jump height from participants wearing both carbon and regular insoles. They would calculate average jump height in carbon and regular insole and be left with data to present to their audience. They may chose to build a graph.
Graphs are a simple and elegant way to express a lot of information. They allow you to visually display the relationship between two (or more) variables. A basic graph typically has two dimensions represented by a vertical line and a horizontal line that intersect at a point called the origin. The horizontal line (x-axis) represents the data from the independent variable (time, frame number, insole type, etc..) where as the vertical line (y-axis) represents the data from the dependent variable (displacement, velocity, acceleration, force, jump height…). The dependent variable is the variable that you are measuring and quantifying.
More often than not, you will put lower numbers at the left on the horizontal axis and at the bottom on the vertical axis. The result is that the graphed line, or the bars in a bar graph, go up—the most natural direction for most data. In reading graphs, however, you should consider the axes and the type of measure being plotted before you interpret the meaning of lines going up and down, just to make sure that “up” represents “more” or “better.”
If your study involves multiple variables, you can represent it in your graph by plotting each level of the variable with a separate line. It is helpful to use different types of lines or different colors. For example, you might graph the reaction time of boys with a solid line and the reaction time of girls with a dashed line. Alternatively, you can use different symbols—say, an X or an O, or a triangle—for the different levels of the variable. You can graph data by hand but in this class, you’ll be asked to use a graphing software called Excel.
Want help Graphing a line-graph in Excel?
Instructions will vary slightly depending on the graph. Begin by entering the data you collected onto an Excel Spreadsheet. You should have at least two columns of data. The first column contains the Independent variable (ex: time, trial number, frame number). The second column, just to the right of the first, should contain the Dependent variable (ex: distance, velocity…). Place the name of the variable at the top of the column and enter the data below. Now you are ready to graph:
- Highlight the area you wish to graph by using the mouse and mouse keys. You should highlight both the ‘Independent Variable’ and one ‘Dependent Variable’ (graph one dependent variable at a time. Click on ‘Insert’ and ‘Chart’.
- On the menu at the top of the page, pick the type of graph you would like (Straight Marked Scatter for line graph).
- In the Chart Layout menu (at the top), type in chart title, x-axis title, y-axis title; make sure to include the units. Remove ALL gridlines.
The graph should have:
- Labeled x and y axis with variable and unit.
- A title
- No gridlines
- No wasted space (adjust the axes value accordingly)
- Clear data lines
Resources
The PHET group in Colorado have a large, and growing, number of simulations to help you with your math basics.
You can check them all out at
https://phet.colorado.edu/en/simulations/category/math/mathconcepts
Play around with this PHET simulation. It will help you see what equations look like.
https://phet.colorado.edu/en/simulation/graphing-lines
For help with analyzing linear graphs, check out
https://phet.colorado.edu/en/simulation/graphing-slope-intercept