In this case, the study is within-subjects with respect to the independent variable Site because each person sees both levels of this variable — that is, both site A and site B. However, the study is between-subjects with respect to Age : one person can only be in a single age group either under or over 30, not both. Well, technically, you could pick a group of underyear olds and wait until they turn 30 to have them test the sites again, but this setup is highly impractical for most real-world situations.
Some independent variables may impose the choice of design. Age is one of them, as seen above. Others are Expertise if we want to compare experts and novices , User Type if we want to compare different user groups or personas — for example, business traveler vs. Outside usability, drug trials are one common case of between-subject design: participants are exposed to only one treatment: either the drug being tested or a placebo, not both.
Unfortunately, there is no easy answer to this question. As seen above, sometimes your independent variables will dictate the experimental design. But in many situations, both designs may be possible. With between-subject design, this transfer of knowledge is not an issue — participants are never exposed to several levels of the same independent variable.
Individual participants bring in to the test their own history, background knowledge, and context. One may be tired after a long night of partying, another one may be bored, yet another one may have received a great news just before the study and be happy. If the same participant interacts with all levels of a variable, she will affect them in the same way.
The happy person will be happy on both sites, the tired one will be tired on both. But if the study is between-subjects, the happy participant will only interact with one site and may affect the final results. Whether your experimental design is within-subjects or between-subjects, you will have to be concerned with randomization, although in slightly different ways. Above, we discussed why randomization is important in within-subject designs: it counteracts the possible order effects and minimizes transfer and learning across conditions.
If the "within" variation is on a par with the "between" variation, the ANOVA test will conclude that the researcher cannot say that the factors had an effect, since any apparent effects could just be due to the random variation which was seen within test groups.
Michael Judge has been writing for over a decade and has been published in "The Globe and Mail" Canada's national newspaper and the U. Michael has worked for an aerospace firm where he was in charge of rocket propellant formulation and is now a college instructor.
Related Articles How to Calculate Significance. The Definition of an Uncontrolled Variable. Five Characteristics of the Scientific Method. What Is a Positive Control in Microbiology? How to Calculate Statistical Significance. What Is the Purpose of Factor Analysis? Difference Between Correlation and Causality. Experiments with Hand Sanitizer. Because a single test participant is assigned multiple user interfaces to test. So, within-subject studies are also more cost-effective than between-subjects studies.
When you recruit test participants for usability studies, there are certain factors you have some control over and others that you have practically no control over. For example, you might be able to recruit test participants that fall under a specific age range or have a particular level of expertise on the subject matter. The benefit of a within-subjects study is that each test participant interacts with each independent variable.
Following our example, this means each test participant will complete the task assigned to them on both websites X and Y. Your usability study test results will likely have significant variance and would be more prone to random noise. You can conduct usability testing studies using either the between-subjects study approach or the within-subjects approach depending upon your goals and resources.
Between-subjects studies or within-subjects studies — which approach do you prefer for user research? Share your thoughts in the comments section below. The modern guide to web accessibility Guide. User Experience 6 minute read by, Rich Staats. There are two possible ways of doing it: Between-subjects study experiment: This study design involves assigning different user interface to different test participant.
This way, each test participant interacts with one user interface. This way, each test participant will test all of the conditions.
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