Developmental research designs are a type of nonexperimental research that can be used in order to study age related changes in behaviour. One principal type of developmental research design is the longitudinal design, in which one group of subjects are studied repeatedly over a period of time. This week I am going to be describing this form of longitudinal design and outlining its strengths and weaknesses.
Within a longitudinal developmental research design, the same participants are observed recurrently over a period of time. This period of time may be quite brief (six months to a year), or very long – sometimes even spanning a lifetime! The researcher may be studying one specific aspect of development (for example, intelligence) or many. The subjects being studied are usually cohort, which means they are roughly the same age and have grown up in similar circumstances. The Longitudinal research design is an example of within-subjects design. However, no treatment is administrated, instead the ‘treatment’ is the age. Longitudinal studies are often described as a number of observations followed by a period of aging and development then another set of observations.
One example of a longitudinal developmental study is that of Howes and Matheson (1992)1, in which the pretend play of a group of 1-to-2 year old children were repeatedly observed every 6 months over a period of 3 years. Howes and Matheson found that complexity of pretend play increases with age and is also a reliable predictor of children’s future social competence with peers.
There are several strengths and weaknesses associated with the longitudinal developmental research design. Firstly, main advantage of the design is that it allows researchers to assess the stability and continuity of several attributes of a sample by repeatedly observing the same participants (Kagan & Moss, 1962)2. Additionally, this type of design also allows researchers to identify developmental trends by looking for common attributes that the subjects share, for example, points at which most children undergo changes (Newman et al, 1997) 3. Another major strength of the longitudinal design is that it avoids cohort effects because the researcher examines one group of people over time, rather than comparing several different groups that represent different ages and generations. Longitudinal research designs also allow researchers to discuss how a single individual’s behaviour changes with age. Finally, this type of design combines both qualitative and quantitative data, creating more in-depth research (Ruspini 1999) 4.
Although so far I have portrayed the longitudinal research design in a positive and beneficial manner, it is important to recognize its limitations and the disadvantages associated with using this form of research design. Firstly, longitudinal research is very time consuming, for everyone involved. The participants must be highly committed in order to continue and complete the duration of the study, and the researcher must remain interested in the research whilst they wait for years to see the final results. As well as being time consuming, this form of research is also very expensive to conduct, since the researchers must track people down and persuade them to come back and participate in the study. Additionally, there is added expense of repeatedly training experimenters to conduct the study if it is going to span over many years. Also, focus of the theory and research into the developmental sciences is constantly changing, thus longitudinal studies may seem new and exciting to start with, but by the end of a 10 or 20 year project the outcome may seem trivial.
Further disadvantages can be identified when assessing the validity of the longitudinal research design. Practice effects may threaten the validity of a study: participants who are repeatedly tested and interviewed may become increasingly familiar with contents. As a result, they may display performance improvements that are unrelated to the normal patterns of development. Furthermore, the longitudinal research design is subject to high drop out rates of participants, which may also weaken the internal validity of such studies. A study conducted by Colby et al. (1983)5 is one example of a longitudinal study to suffer participant attrition: participants were interviewed at intervals of 3 to 4 years over a 20 year period and the sample size decreased from 84 participants to only 51. Since longitudinal studies may go on for a number of years, participants may lose interest in the study, move away or die. When participants drop out of a study, it is known as participant attrition, which can result in smaller and non-representative samples. The participants who drop out may have different attributes as those who stay, consequently, the group at the end of the study may have completely different characteristics as the initial group at the beginning. For example, if the less-motivated participants drop out during a study, the group of participants at the end will be more motivated than the group at the start and this higher level of motivation may explain the changes made over time, rather than the age.
One final limitation of the longitudinal developmental research design is the cross-generational problem. As I mentioned earlier, children in a longitudinal study are usually drawn from one cohort, therefore they are likely to have very different experiences than children from other eras. For example, consider how much change there has been since the 1940s and 1950s, when children in some of the early longitudinal studies were growing up. Family dynamics have changed drastically since then, and modern families are a lot smaller now, move houses more frequently and often use day-care centers and nurseries. Children now have access to computers, video games, and televisions, which were not available in the 1940s and 1950s. Children of earlier eras grew up in a completely different world, so it cannot be certain that children today develop in exactly the same way. As a result, these cross-generational changes may limit the conclusions of the longitudinal studies to only the participants growing up whilst the study was in progress.
In conclusion, in the longitudinal developmental research design, the same group of participants are observed and measured at different intervals over a period of time, thus cohort effects are not a problem. Stability, continuity and normative trends can easily be identified and quantitative methods are combined with qualitative methods. However, I believe the weaknesses of this type of research design outweigh the strengths. Longitudinal studies are very time consuming for both researchers and participants, as well as being extremely expensive. The validity of such studies can be questioned due to practice effects, and participant attrition may create biased and non-representative samples. Finally, cross-generational problems can make it difficult to generalize finding from studies between different eras. In all, researchers must take into account the many disadvantages of using this type of research design before undertaking a study. However, these disadvantages may be overcome by using a very similar research design called the sequential design, which combines the best features of longitudinal research. I will discuss this design further next time.
- Howes, C., & Matheson, C. C. (1992). Sequences in the development of competent play with peers: Social and social pretend play. Developmental Psychology, 28, 961-974.
- Kagan, J., & Moss, H. A. (1962). Birth to maturity. New York: Wiley.
- Newman, D. L., Caspi, A., Moffitt, T. E., & Silva, P. A. (1997). Antecedents of adult interpersonal functioning: Effects of individual differences in age 3 temperament. Developmental Psychology, 33, 206-217.
- Colby, A., Kohlberg, L., Gibbs, J., & Lieberman, M. (1983). A longitudinal study of moral judgement. Monographs of the Society for Research in Child Development, 48, 1-2.