Between-Subject Design Vs Within-Subject Design

To begin this semester blogs, I thought I would discuss the two basic research designs concerned with psychological research, those being between-subject designs and within- subject designs. Within this blog I am going to disregard the fact that the majority of the time we choose a design based on our hypothesis. I am going to look more into the pros and cons of both and decide despite my research hypothesis, which one in general appears to be the better design, resulting in more valid findings.

Firstly I shall discuss between-subjects designs; this is where different groups of individuals are compared. Therefore the researcher manipulates the independent variable, which creates different treatment conditions; separate groups of participants are then assigned to each of these different conditions. The dependent variable is then measured for each individual and differences are looked for between-groups.  When given the basic description of a between-groups design, my initial thought is the scores are independent of each other and therefore I can be confident that they are clean results and have not be affected by practice effects, or fatigue, boredom or contrast effects. However in the past when thinking about conducting a between-groups experiment, I realise a lot more participants will be need than if I were to conduct a within-subjects design. As each participant is only giving one piece of data, if I were to conduct a study with 3 conditions needing 20 participants in each one, I would need 60 participants whereas within groups I would only need 20. This may prove an issue in situations where a population is hard to get hold of. Despite the advantages of different individuals being used in conditions, this is also a primary disadvantage, as it leaves the research open to the problem of individual differences. For instance if I were to conduct a study with my friend and boyfriend both participating,  they differ in age, IQ, personality, interests but also the day before my experiment my friend may have had a sleepless night whereas my boyfriend slept soundly. Within my research the major concern for me would be these individual differences turning into confounding variables, basically is my finding actually because of the treatment or because of a confounding variable such as age. Sometimes we can just not tell.

Before choosing my winner, I shall discuss within- subjects designs, this is where a single group of participants are tested or observed in all of the different treatments being compared. Therefore the same group of individuals participate in all different levels of the independent variable. Again when given the basic description of within- groups design, my first thought is this is the complete opposite to between subjects in regards to confounding variables, at the very least they will be reduced. There are no individual differences to confound the results; the individuals in treatment 1 are exactly the same as treatment 2. You can more than likely conclude differences are due to the treatment rather than individual differences. Furthermore if individual differences are consistent across treatments, they can be measured and removed from the rest of the variance of the data. Despite its advantages, one major setback of within-subjects is that participants go through different treatments each administered at a different time, therefore time-related factors may come into play, such as fatigue and weather; also issues such as history, maturation, instrumentation, testing effects and regression hold disadvantages. Lastly such design is subject to participant attrition, where participants may simply just drop out, losing you valuable data.

After discussing both designs it is clear that both hold advantages and disadvantages, and it is apparent that the advantages of one design are essentially the same as the disadvantages for the other design. Therefore I have come to the conclusion that no design is better in general than the other in regards to validity; as when each design is faced with its disadvantages, each disadvantage is offered a solution. For instance between-groups main disadvantage is individual differences, this can be improved by establishing equivalent groups by random assignment or matching, also variance can be minimized by standardizing procedures and keeping the treatment setting stable. Lastly within-groups main disadvantage is order effects, dealing with such problem can be done through counterbalancing the conditions. No winner here guys :)

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5 thoughts on “Between-Subject Design Vs Within-Subject Design

  1. hey i really like your blog, you have made it really easy to read! I like how you have seperately talked about within and between subject designs and made a lot of effort in explaining them clearly. This really helped me because i dont really understand the differences and you made it make sense in a simpler way, which is brilliant. I think you could have made your conclusion a little longer and perhaps brought in some research demonstrating the use of within and between group variances. :)

  2. I agree with the conclusion you have come to, in that neither design is best overall, rather one or the other may be better suited to a specific situation and experimental design. Many of the disadvantages of each design can be overcome using techniques such as counterbalancing and matching, increasing the benefits of using each. One example where either design may be used could be during a drug trial. Within subjects design would mean that individual differences influencing he effects of the treatment would be eliminated, however order effects such as experience may act as a confounding variable. However, a between subjects design would eliminate individual differences, though not being able to examine individual effects may also be a disadvantage. Therefore, I would come to the same conclusions you have, neither is a better design.

  3. I think this is a very interesting topic, and you have covered many relevant points such as the issues of order effects of a within-subjects design and the problems with individual differences when conducting a between-subjects design. However, I feel that you have disregarded a very important idea at the start of your blog, and that is why researchers choose their design. Many research studies call for a specific sort of design, for example when attempting to find the effectiveness of a treatment on a mental illness using a within-subjects design would not be feasible as you cannot place a person under treatment and then also in a control group. Similarly there are times where a between-subjects design would be inappropriate, for instance when looking at development of children over a period of time, a longitudinal study, within-subjects design would be the most appropriate.

    So, to conclude I feel that you have raised many interesting points, however, comparing the two designs is often irrelevant in research as the design of the study is set out by the ideas behind the study.

  4. Pingback: Comments for my TA, blog 1. « laurenpsychology

  5. i think whether children can be used in psychology experiment depends intirely on the context and subject bbasis of the student cfor example innocent shpuld nnever as children have to endure the stress felt those in zimbardos or asches study but on the other hand they can be veryu useful in eduacational or developmental studyb as they are still deverloping thus can be taught and monittered for effect and reaction for example Jerome Bruner studied how children learn language as a process at oxford university in 1983;studies such as this which show clear develpment would be almost pointless to do with an adult unless it was a longitudunal study, and perfect with children at a prime developmental age. http://clt.sagepub.com/content/1/1/111.short

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