READ: Experiments and Variables

READ: Experiments and Variables

It’s exciting to roll down a skateboarding ramp, especially if you’re going fast. The steeper the ramp, the faster you’ll go. What else besides the steepness of a ramp influences how fast an object goes down it? You could do experiments to find out.

An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values. For example, the speed of an object down a ramp might be one variable, and the steepness of the ramp might be another.

Experimental Variables

There must be at least two variables in any experiment: a manipulated variable and a responding variable.

• A manipulated variable is a variable that is changed by the researcher. A manipulated variable is also called an independent variable.
• A responding variable is a variable that the researcher predicts will change if the manipulated variable changes. A responding variable is also called a dependent variable.

Q: If you were to do an experiment to find out what influences the speed of an object down a ramp, what would be the responding variable? How could you measure it?

A: The responding variable would be the speed of the object. You could measure it indirectly with a stopwatch. You could clock the time it takes the object to travel from the top to the bottom of the ramp. The less time it takes, the faster the average speed down the ramp.

Q: What variables might affect the speed of an object down a ramp?

A: Variables might include factors relating to the ramp or to the object. An example of a variable relating to the ramp is its steepness. An example of a variable relating to the object is the way it moves—it might roll or slide down the ramp. Either of these variables could be manipulated by the researcher, so you could choose one of them for your manipulated variable.

Controlling Variables

Assume you are sliding wooden blocks down a ramp in your experiment. You choose steepness of the ramp for your manipulated variable. You want to measure how changes in steepness affect the time it takes a block to reach the bottom of the ramp. You decide to test two blocks on two ramps, one steeper than the other, and see which block reaches the bottom first. You use a shiny piece of varnished wood for one ramp and a rough board for the other ramp. You raise the rough board higher so it has a steeper slope (see sketch below). You let go of both blocks at the same time and observe that the block on the ramp with the gentler slope reaches the bottom sooner. You’re surprised, because you expected the block on the steeper ramp to go faster and get to the bottom first.

Q: What explains your result?

A: The block on the steeper ramp would have reached the bottom sooner if all else was equal. The problem is that all else was not equal. The ramps varied not only in steepness but also in smoothness. The block on the smoother ramp went faster than the block on the rougher ramp, even though the rougher ramp was steeper.

This example illustrates another important aspect of experiments: experimental controls. A control is a variable that must be held constant so it won’t influence the outcome of an experiment. The control can be used as a standard for comparison between experiments. In the case of your ramp experiment, smoothness of the ramps should have been controlled by making each ramp out of the same material. 

Q: What other variables do you think might influence the outcome of your ramp experiment? How could these other variables be controlled?

A: Other variables might include variables relating to the block. For example, a smoother block would be expected to go down a ramp faster than a rougher block. You could control variables relating to the block by using two identical blocks.

Replication 

Did a math teacher ever tell you to check your work? If you’re adding numbers, that means repeating the calculation to see if you get the same answer the second time. If you get the same result twice, then the answer is probably correct. But if you get a different answer the second time, at least one of the results must be incorrect. Then you have to add the numbers a third time and hope that the third answer will agree with one of the other two.

Scientists also have to “check their work.” The results of an investigation are not likely to be well accepted unless the investigation is repeated—usually many times—and the same result is always obtained. Getting the same result when an experiment is repeated is called replication. If research results can be replicated, it means they are more likely to be correct. Repeated replication of investigations may turn a hypothesis into a theory. On the other hand, if results cannot be replicated they are likely to be incorrect.

Why Replication is Important in Science: An Example

The following example shows why replication is important in science. In 1998, a British researcher published an article in a medical journal reporting that he had found a link between a common childhood vaccine and autism (see Figure below). According to the article, children in his study developed autism soon after receiving the vaccine. Following publication of the article, many parents refused to have their children vaccinated. Several epidemics occurred as a result, and some children died of the diseases.

Soon after the original study was published, other researchers tried to replicate the research. However, it could not be replicated. No other studies could find a link between the vaccine and autism. As a result, scientists became convinced that the original results were incorrect. Eventually, investigators determined that the original study was a fraud. They learned that its author had received a large amount of money to find evidence that the vaccine causes autism, so he faked his research results. If other scientists had not tried to replicate the research, the truth might never have come out.

Summary

• An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values.
• There must be at least two variables in any experiment: a manipulated variable and a responding variable.
• A control is a variable that must be held constant so it won’t influence the outcome of an experiment.
• Getting the same result when an experiment is repeated is called replication. If research results can be replicated, it means they are more likely to be correct.
• Replication is important in science so scientists can “check their work.” The result of an investigation is not likely to be well accepted unless the investigation is repeated many times and the same result is always obtained.

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