What type of experimental variable is affected by the manipulation of the independent variable?

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What is a Dependent Variable?

In an experiment, the independent variable is manipulated and the effects observed. These observed effects are called dependent variables. They are often the hypothesized outcome of manipulating the independent variable.

A change in the dependent variable depends on the independent variable, hence the name. The dependent variable responds to the independent variable, and it’s this relationship that researchers attempt to measure when conducting experiments.

A well-designed experiment normally incorporate one or two independent variables, with every other possible factor eliminated, or controlled. There may be more than two dependent variables in any experiment.

Dependent Variable Examples

A researcher might wish to establish the effect of fertilizer on the rate of plant growth; amount of fertilizer is the independent variable. They could regard growth as height, weight, number of fruits produced, or all of these. A whole range of dependent variables arises from one independent variable.

For an example, consider pharmaceutical trials, where researchers may administer new medicines to participants to determine the optimal dose. The independent variable is how much medicine the participants receive. If the medicine is an antihistamine, the dependent variable might be the severity of allergy symptoms a participant displays, such as red eyes or sneezing.

Here, the researchers might also measure other relevant dependent variables which may turn out to be unwanted side effects of the medicine, such as drowsiness.

In any experimental design, the researcher must determine that there is a definite causal link between the independent and dependent variable.This reduces the risk of 'correlation and causation' errors. Controlled variables are used to reduce the possibility of any other factor influencing changes in the dependent variable, known as confounding variables.

In the above plant growth example, the plants must all be given the same amount of water, or this factor could obscure any link between fertilizer and growth.

For the antihistamine trial, a confounding variable may be that a participant’s symptoms could improve simply with the passage of time. This is addressed with a control group that receives no medicine at all, allowing researchers to compare all groups and isolate only the true effects of the medicine. The participants might also be asked to stop all other medication during the experiment – another possible confounding variable.

The relationship between the independent variable and dependent variable is the basis of most statistical tests, which establish whether there is a significant correlation between the two. The results of these tests allow the researcher to accept or reject the null hypothesis, and draw conclusions.

Independent Variable or Dependent Variable?

It can sometimes be tricky to tell the independent and dependent variable apart. If you’re unsure:

• Ask yourself which variable is affecting or influencing which other variable. The variable being affected is likely the dependent variable.

• Try to construct a cause-and-effect sentence with the independent variable being the cause and the dependent variable being the effect. For example, “coffee consumption causes increased productivity.” The sentence will not make much sense if you’ve confused the independent and dependent variables: “productivity causes increased coffee consumption.”

• Ask which outcome variables you are measuring – these are likely the dependent variables.

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What is a Variable?

Any factor that can take on different values in an experiment is a scientific variable.

For example, in an experiment investigating the effectiveness of a new training program, the variables might be:

• Final test scores
• Student age
• Time spent on the training program
• Time to complete final test
• Student gender
• Student ratings of the training program 

Depending on how the researcher operationalizes all the variables in an experiment, the above could be either dependent or independent variables.

It’s the research design that decides which variables are manipulated and which are measured as a result of that manipulation.

What is the Independent Variable?

The independent variable is "independent" because its variation does not depend on the variation of another variable in the experiment/research project. The independent variable is controlled or changed only by the researcher. This factor is often the research question/hypothesis behind the outcome of the experiment.

In the above example, the researcher may have wanted to see if participating in the training program raised students' scores on a final test.

Mini-quiz 1

Can you identify the independent variable in this experiment?

1. Score on the test
2. Time spent on the training program
3. Participation on the training program

What do you think is correct? The answer is at the bottom of the article.

How Many Independent Variables Do You Test?

There are often not more than one or two independent variables tested in an experiment, otherwise it is difficult to determine the influence of each upon the final results. There may be several dependent variables, because manipulating the independent variable can influence many different things.

For example, an experiment to test the effects of a certain fertilizer on plant growth could measure height, number of fruits and the average weight of the fruit produced. All of these are valid analyzable factors arising from the manipulation of one independent variable, the amount of fertilizer.

Potential Complexities of the Independent Variable

The term independent variable is often a source of confusion; many people assume that the name means that the variable is independent of any manipulation. The name arises because the variable is isolated from any other factor, allowing experimental manipulation to establish analyzable results.

A useful acronym is DRY-MIX. This helps you remember which axis to plot your data should you need to draw a graph:

• D - Dependent
• R - Responding
• Y - Y-axis

• M - Manipulated
• I - Independent
• X - X-axis

Some research papers appear to give results manipulating more than one experimental variable, but this is usually a false impression.

Each manipulated variable is likely to be an experiment in itself, one area where the words 'experiment' and 'research' differ. It is simply more convenient for the researcher to bundle them into one paper, and discuss the overall results.

The researcher above might also study the effects of temperature, or the amount of water on growth, but these must be performed as discrete experiments, with only the conclusion and discussion amalgamated at the end.

Examples of the Independent Variable

Jane Elliott's Anti-Racism Experiment

Third grade teacher Jane Elliott’s famous experiment involved dividing her class into two groups: blue-eyed and brown-eyed children. She gave the blue-eyed children extra privileges and emphasized how superior they were to the brown-eyed, who were now a “minority group.”

As a result, the brown-eyed children saw a drop in confidence, academic performance and an increase in bullying. However, when she later labelled the blue-eyed group as the inferior, these effects were reversed.

Here, the independent variable was group status, i.e. whether the children where in the privileged group or not. This had various observable effects on the children. Importantly, the eye color of the children was not the independent variable here. Eye color was an arbitrary choice made by the teacher to draw parallels to racism and prejudice.

Mini-quiz 2

Can you identify a possible dependent variable in this experiment? 

1. Level of bullying
2. Academic performance 
3. Confidence level
4. All of the above

What do you think is correct? The answer is at the bottom of the article.

Bandura Bobo Doll Experiment

In the Bandura Bobo Doll experiment, whether the children were exposed to an aggressive adult, or to a passive adult, was the independent variable.

This experiment is a prime example of how the concept of experimental variables can become a little complex. Bandura also studied the differences between boys and girls, with gender as an independent variable. Surely, this is breaking the rules of only having one manipulated variable!

In fact, this is a prime example of performing multiple experiments at the same time. If you study the structure of the research design, you will see that the Bobo Doll Experiment should have been called the Bobo Doll Experiments.

It was actually four experiments, each with their own hypothesis and variables, running concurrently. It would have been expensive, and possibly unethical, to test the children four times and, if the same children were used each time, their behavior may have changed with repetition.

Careful design allowed Bandura to test different hypotheses as part of the same research.

Mini-quiz 3

Can you identify the separate independent variables in this experiment? Pick two.

1. Presence or absence of Bobo doll
2. Gender of the role models
3. Aggressiveness of the role models
4. Number of children

The answer is at the bottom of the article.

Mini-quiz Answers:

Mini-quiz 1

Can you identify the independent variable in this experiment?

Option 3. Participation on the training program.

The researcher could manipulate the variable of whether students participated on the program or not, then measure the results, for example their score on a final test.

Mini-quiz 2

Can you identify a possible dependent variable in this experiment?

Option 4. All of the above.

The experiment measured the children's overall behavior. But this could have been broken into separate dependent variables, for example academic performance, level of bullying, or confidence levels. 

Mini-quiz 3

Can you identify the separate independent variables in this experiment? Pick two.

Option 2 and 3. The gender of the role models and the aggressiveness of the role models.

Bandura was interested to see if a child would imitate their role model, but he also wanted to see if a child was more likely to imitate them if they were of the same gender.

Which variable is affected by manipulation?

What is the independent variable manipulative of the experiment?

What are the independent variables The variables that were manipulated?

What type of variable that is manipulated in an experimental study?