This time of year, most of the questions I get focus on how to graph the data collected. If you are trying to decide between a line graph and a bar graph… it usually is not your choice. The type of data and type of project dictate what, mathematically, you are “allowed” to do.
Here is a reminder:
LINE GRAPH – you must have quantitative and continuous data for both your independent and dependent variables. That is data that can be assigned a number that could be logically placed on a number line. For example, temperature, time, size, weight, wavelength, speed, etc. HOWEVER – if either of your variables are qualitative you CAN NOT use a line graph (even though your graphing program will let you!!). This is one of the biggest mistakes I see at science fairs.
BAR GRAPH – the overwhelming majoring of science fair projects are best presented with a bar graph. Usually that means you have a qualitative variable on the x-axis (horizontal) and a quantitative variable on your y-axis (vertical). Use bar graphs for all projects that have different “treatments”, for example: different conditions under which you pop popcorn, or freeze liquid, or bounce balls, or break eggs, or grow plants, or fly paper airplanes, etc. Use bar graphs too if you are varying colors (e.g. red vs. blue) or groups (e.g. male vs. female; different types of soda; different types of seeds; etc.).
With yes/no data (or counts):
—You might be able to make a pie chart showing the percentage of yes vs. no or
—You might be able to make a bar graph by calculating the number of yes (or no) divided by the number of attempts (or trials) and then you have a number for your y-axis. For example if you ask 20 women and a question and 10 answer yes and ask 20 men the same question and 5 say yes; then you could calculate the percentage of women who answered yes (10/20*100 = 50%) vs. the percentage of men (5/20*100 = 25%) and graph the percentage.
—The same applies to projects where you have “counted” the occurrence of something, for example the number of times the can floated; or flower died; or egg broke. Convert “counts” into percentages and make a bar graph or pie chart depending on your project.
DON’T FORGET TO LABEL BOTH AXES with labels, units and numbers. That is another very common mistake.