4.5+How+Many+Variables?

by Phil Kahler
 * How Many Variables? **

Students often fall into the trap of thinking that in order for an experiment to be good, it must be complicated. Still other students complicate their experiment because they have not fully thought through the cause and effect relationship of their independent and dependent variables. If your investigation has more than one independent variable and/or more than one dependent variable chances are pretty good that you still need to focus and simplify things.

Simple experiments are better because they are focused. They are easier to:
 * Keep It Simple**
 * Conduct
 * Analyze the Data
 * Draw Conclusions

So here are two simple rules that I give my students: 1) Identify only one Independent Variable 2) Identify only one Dependent Variable

Here is why: Having only one independent variable and one dependant variable forces you to think about the cause and effect relationship between the two.

The **independent variable** is the one thing that is changing or has been manipulated in some way. The independent variable is going to cause a response. It has an effect on your study population.

The **dependent variable** is the response. It is what your study population does because of the independent variable. This is what you observe happening.

I require my students to write a **“formalized” hypothesis** for their experiments. Again, this forces my students to think about the cause and effect relationship between their independent and dependent variables. All formalized hypotheses begin with the word “if”, and contain the word “then” somewhere in the middle of the sentence.

For example: “If the temperature is 5°C or less, then more Black-capped Chickadees will be observed at the feeders.” Temperature is the independent variable. Number of Black-capped Chickadees is the dependent variable. Here we expect to discover the relationship between the temperature and how many Chickadees we see at the feeders.

When it comes time to graph the data from this experiment we can simply plug in our temperature and bird numbers on the “Y” axis. Date range or time will show up on the “X” axis.
 * Think Ahead:** How do you want to graph your independent and dependent variable?



DataTable arranged into columns so that it could be sorted by temperature.

Graph with assending temperature data.

With scatter plot you see temperature on the x axis at the bottom. The only line is for number of Chickadees. It is interesting to see what happens when you have different numbers of Chickadees observed for the same temperature.


 * Disclaimer: ** //By the way, none of the data in my examples on this page is real. I made all of these numbers up just to see how Excel would respond.//