Table of Contents

• Prequiz...
• 1: The Case of the Confusing Axes
• 2: Choose your x and y carefully
• 3: Practise labelling the axes
• 4: Next step: ranging the axes
• 5: Practise ranging the axes
• 6: Next step: scale the axes
• 7: Practise determining labelled interval distance
• 8: How to scale axes
• 9: Finally, fill in the axes
• 10: Summary

Choose your x and y carefully

Scientists like to say that the “independent” variable goes on the x-axis (the bottom, horizontal one) and the “dependent” variable goes on the y-axis (the left side, vertical one).

When you're talking about variables, “independent” means that the researcher (you or someone else in a white coat) can pick any value you want for that variable. Using the TV viewing distance data, you can imagine the researcher putting little pieces of tape on the floor and positioning her small experimental subjects at just the right distance ... whereas I can't think of any way the researcher can directly control how much eyesight each child has lost.

In this example:

•  The x variable is how far the child sits from the TV

•  The y variable is how much eyesight she loses

OK, now you try: for each pair, which is "x" and which is "y"?

OK, those two were easy. In both cases, the x variables (temperature of tank or amount of food given) are easily controlled by the experimenter. Sometimes the "x" variable can't actually be controlled but only "chosen" by researcher -- this is especially the case when some version of "time" is the x variable.

However, be careful. Just because a variable includes time does not mean that it is automatically the x variable. Sometimes the amount of time a process takes depends on a treatment, and then it's the "y" variable.

Did you get the Dilaudid example? In this case, the researcher is trying to find out how long it takes to get pain relief; this depends on how much medication is given. The researcher can directly control (or choose) dosage, but not the time.

Finally, data tables often contain many columns, so it's important to figure out which ones belong on a graph. Below, type "n" for data that will NOT be represented by either "x" or "y".

What’s x, what’s y, and what should NOT be used in a graph?