Introduction to Biology: Experimental Variables

Dependent Variables

I like to start with the dependent variable, although everyone tends to start with the independent.  The dependent is pretty straight forward because it is usually the data that you collect.  When we write about experiments, students often forget to describe this one.  You always tell me about the set-up, but not about the data collection!

Let’s look at some of these hypotheses to understand.  How might you go about testing the statement that corn plants grow taller with fertilizer.  You might grow corn in different concentrations of fertilizer like 20% and 40% and so on.  That makes sense.  And then, you might record heights after letting them grow for a certain amount of time.  What’s the data you collect?  What do you measure?  Yes!  The heights of the plants is the dependent variable.  This data will allow you to say: Yes, corn plants grow taller with fertilizer.  Or not, as the case may be. 

Let’s skip to the third hypothesis here.  What is the data you are collecting? Price!  Yes!  This is what will indicate which jacket is cheaper.  Before we do the remaining hypothesis, let’s recognize something.  Neither of these two hypotheses here tell you that height or price is the dependent variable.  You had to think a little bit about how you would test these.  Keep this in mind for the last one.  What might be the data we could collect for this one?  You might be saying: well, I would measure the enzyme activity.  Yeah, I’ll take that.  It’s vague, but without really thinking about an experiment, we can’t get more specific.  For example, lactase digests lactoseA disaccharide sugar found in milk. produce glucoseA simple sugar that is the main source of energy for cells..  You could measure the glucose.  But that’s so specific!  This is why defining variables can be difficult.  They are vague sometimes. 

Independent Variables

OK, so corn plants grow taller with fertilizer.  I’m not debating this with you, it’s just an example.  What are we testing?  You might say corn plants.  OK, but wrong.  Let’s think of what we might do to test this statement.  We might grow corn in two fields and use fertilizer in one of them and not the other.  You are varying and manipulating the fertilizer.  Right?  That is what we are testing, the effect of fertilizer. 

Let’s think about the next statement because practice is the way to understand the independent variable.  We could use lactase, which is the enzyme that digests lactose to test this.  You might put lactase with some lactose in different temperatures and measure something to indicate the activity.  What are you intentionally manipulating to be different?  The temperature!  Yes! 

The Control and Controlled Variables

Controlled variables are usually easy for students to recognize.  There are usually many controlled variables. They remain constant, like using the same corn plants, the same enzyme, and the same jacket.  The problem with controlled variables is that they get confused with what is called the control.  So, the control is different than the controlled variables.  Please know that right at the outset here.  The control is a basis of comparison.  This is really hard to define for some experiments.  Let’s start with easy ones.  We might amend this first experiment with corn plants so that we are growing plants with different concentrations of fertilizer.  We would need some plants grown with no fertilizer as a basis of comparison.  Plants not grown with fertilizer are the control.  Hmmmm…this one is tough.  What do we designate as the control for temperature?  0?  Room temp?  You get to decide, but you better be pretty clear about it.  How about this…set up this experiment by combining the enzyme with some other goo. Ensure it all starts out at the same temperature.  This is the basis of comparison.  I know a bit tougher.  How about this one.  Are you thinking about a placeboA substance with no therapeutic effect used in control groups. group, or a group that you don’t give Drug X?  Yes.  So, a bit more confusing.  When you encounter these concepts in later labs, please come back to this presentation. Refer to it for guidance. Comparing your experiment to these examples can help you.  As we went through these variables, the definitions don’t help, right?  The examples help.