This design has three distinct features: groups are selected based on current differences instead of random allocation, relies on existing differences instead of change following intervention, and uses no time dimension. The design only measures differences from or between a variety of phenomena, subjects or people rather than a change process. Therefore, the researchers who use this design employs only a relative passive approach in making causal inferences centered on findings. For example, in the research conducted by Von Kries, et al. (1999) on the impact of breastfeeding and the risk of obesity, routine data was collected on weight and height of 134,577 children aged between 5 and 6 at the time of school entry. This study assumes all the other ages of children and takes this as a representative of the whole age group.
Longitudinal Study Design
This type of design follows a similar sample over time by making repeated observations. According to Afidep (2016), this design describes change patterns and establishes the magnitude and the direction of causal relationships. For example, in the article by Jessor R and Jessor Sl (1977), the problem behavior and psychosocial development of the youth was conducted in phases. This study plots trajectories of change over time in behavior, social environment and personality. The researchers take measurement on every variable over two or more divergent time periods. Through this, a researcher can be able to measure variable change over time.
Both cross-sectional and longitudinal research designs use observational study. The two can be used in a study of age related changes. Similar with cross-sectional design, longitudinal design results in the problem: confusing age with another cohort. These results to a problem of lack of identification of whether the results obtained are due to age or other variables. Both study designs requires accurate sampling and a large sample size to reach representativeness.
As Afidep (2016) explains, cross-sectional studies are quick and inexpensive to conduct as researchers can test many people with different ages at a given time. It is also the best method of studying age differences, and not the changes of age. However, this design does not provide a very rich picture of development, as it only examines one small group of people at a given time. Finally, when using this design, it is difficult to compare one group with another because distinct to longitudinal design, participants do not act like their own control. Cross-sectional design collects data from a whole population and it is thus applicable while conducting a research in a large population.
In contrast to cross-sectional design, longitudinal studies permit the measurements of changes or differences of a variable from one period to another. In contrast to longitudinal studies, cross-sectional design according to Afidep (2016) focuses on drawing and studying inferences from existing differences between phenomena, subjects and people during an active intervention by researchers to measure and produce change. Unlike longitudinal design which takes a sample from small group, cross-sectional studies estimates the prevalence of interest outcome due to the fact that the sample is taken from the whole population. This fact is an added advantage to the cross-sectional design as it is capable of using data from different subjects. Longitudinal study design is geographically bound unlike cross sectional study which is not.
Cross-sectional study would be practical for a large study as it presents the whole population. On the other hand, longitudinal design is practical to a small study as it uses a small sample size and is geographically bound. Longitudinal design is more superior to cross-sectional as it describes change patterns and establishes the magnitude and direction of causal relationships. However, cross-sectional design uses short time as it discusses situation over time unlike the longitudinal which uses continuous studies thus making it more expensive in long run.
Afidep,. (2016). Evidence -‐ Informed Policy -‐ Making Training Curriculum 112 Major Types of Research Designs – Module 3 Handout 1. University Of Southern California Libraries. Retrieved from https://www.afidep.org/wp-content/uploads/2016/08/Module-3-Handout-1-12-Major-Types-of-Research-Designs.pdf
Jessor R, & Jessor Sl,. (1977). Problem behavior and psychosocial development: a longitudinal study of youth. Popline. Retrieved from http://www.popline.org/node/499076
von Kries,, Koletzko, Sauerwald, Mutius, Barnert, Grunert, & Voss,. (1999). Breast feeding and obesity: cross sectional study. Bmj. doi: https://doi.org/10.1136/bmj.319.7203.147
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