Explaining the “controlled variables” is tricky. Most students state or maybe describe, but few really explain them. Here’s how to do it and an example to show you.
What are controlled variables?
A “controlled variable” is any factor that could affect your results so you kept it constant in both conditions of your experiment. You want to control certain variables so they don’t become confounding variables.
Some common variables that might affect results and so need to be controlled are:
- Time of day
- Environment (e.g. lighting, distracting noises, etc.)
- Participant characteristics (e.g. gender, age, language, handedness education, occupation, etc.)
- Materials (e.g. word lists, reading passages).
- Instructions (e.g. standardized instructions)
- Ethical consideration forms
- Research bias (controlled with single or double-blind designs and random allocation)
- IA Tips: How to explain your…PARTICIPANTS
- IA Tips: How to explain your…MATERIALS
- How to avoid the biggest mistake in IAs
Describe vs. Explain
Describe v. Explain is the difference between mid-marks (1-2) and top marks (3-4) in the Exploration. Students will often get feedback saying their controlled variables are “described” but not “explained.” What does this mean?
Always explain, never just describe – the advice is as relevant for the IA as it is for exams.
It means there’s a summary of how the variables have been kept consistent in both conditions, but there’s no reason why it’s necessary to do so.
The list above contains many factors that are explained in other sections of the Exploration, such as participant characteristics and materials. If you’ve explained controlled variables in your participants and materials, simply write “See participant characteristics and materials for more explanations of controls” or something like that to show you’ve done it).
Random Allocation: This is a key component of any experiment. If you’re using independent samples then random allocation is straightforward. For repeated measures, you can randomly allocate the order of conditions your participants experience (e.g. randomly select who goes Condition A then B, and who has B then A). This helps control for participant variability and researcher bias.
How do you “explain” them?
Give a reason why the variable had to be controlled as well as how you did it. Explain how the variable could affect your specific results and thus warrants special attention to keep constant.
Example: Time of Day
The most obvious variable to control is the time of day the experiment is conducted. For example, if one condition is done in the morning and another in the afternoon, it could be the difference in day that explains the results not the independent variable. This is especially relevant when doing studies in cognitive psychology (e.g. Baddeley et al. 2007 found that immediate memory is better in the morning than the afternoon).
Common Student Answer: We made sure we did our experiment at the same time of day for both groups so our results wouldn’t be affected.
This states how the variable was controlled, but the reason given for why it needs to be controlled is too vague to be useful.
Excellent Student Answer: We made sure we did our experiment at the same time of day (between 10am-11am) for both groups. We did this to make sure attention and concentration weren’t influenced since people tend to get drowsy after eating. Studies have also shown what we eat can affect concentration, which might affect participants’ scores on the memory test (e.g. Madison et al. 2020) so we avoided this issue by having all participants do the experiment before eating lunch.
This answer states how the variable was controlled. It also goes further to give a specific reason why it needs to be controlled for this particular study.
Is evidence always necessary?
You don’t need a study to support every point you make in your IA Exploration. Sometimes the reason is logical and doesn’t need empirical evidence. For instance, you might decide to do both conditions at the same time to avoid contamination from one group discussing with the other. No study needed to show this potential problem since it’s logical.
However, it really helps lift your “controlled variables” section from description into exploration for some factors if you cite a study justifying your explanation. If you’re aiming for a 7, perhaps try to find at least one study to support your explanation of one controlled variable. (Here’s the study by Madison referenced above, or Baddeley’s might also be relevant).
The extra detail needed to “explain”all aspects of the exploration is why I recommend this section of the IA is the longest, at about 600-800 words.
I recommend choosing two or three of the most important controlled variables than explaining those. Your choices will depend on your study.
Travis Dixon is an IB Psychology teacher, author, workshop leader, examiner and IA moderator.