The longitudinal approach has lots of merits, particularly since it controls individual differences. However it is known for being time consuming. That is to say in Schaffer and Emerson’s study of attachment, data was collected monthly. For example data is collected at 7 months followed by a one-month waiting period before data is collected once again at 8 months old. It has been known for the longitudinal approach to take years. An example of this could be the study of children’s memory at the age of five and ten. Data is collected when the child is aged five. Following this there is a five-year waiting period before data collection is complete at age ten. There are problems, however, regarding the longitudinal approach. Because of the issue of time and the long waiting periods, participant drop out, also known as participant attrition, tends to be high. Participant attrition occurs for various reasons such as moving away, a lack of interest in the study or on a more serious note the death of a child. The problem of participant attrition raises the question of whether the remaining participants are representative of the original sample. It has been known for studies that start out with one hundred participants can end up with as little as thirty-five participants resulting in a participant attrition rate of 65%.
A further disadvantage is that of the test carried out. It is possible for it to be revised over time to include new items with new norms. The question of whether to use the original or revised test is raised. Furthermore environmental changes may occur over time such as changes in education, for example the approaches to teaching and the implementation of the national curriculum. Development of this kind may affect the comparison of data taken from a five year old to that taken from a ten year old resulting in less meaningful and distorted findings. In spite of these problems the longitudinal design is useful for giving detailed descriptions of changes and raising questions with regards to the process of development. Nonetheless there is an alternative approach that also examines change with age.
The cross-sectional approach, which uses different children at the same age point, is known for being quick; data can be collected in a matter of hours regardless of the age points. The cross-sectional approach is also cost effective and comparisons can easily be made between age groups. Nevertheless one major problem with this approach is the cohort effect whereby children at different age points are likely to have different histories. An example of this is a cross-sectional study by Weschsler (1936), which illustrates a cohort effect. He conducted an intelligence test collecting data from adults from the age of twenty to mature adults. Weschsler found that young adults scored higher than middle-aged adults who scored higher than mature adults. On appearance this indicates that intelligence declines with age, however other factors may have affected the performance of the three age groups. That is to say mature adults may have experienced fewer years of formal education than young adults and the same may apply to middle-aged adults. Therefore Weschsler study does not represent change with age but a cohort effect. Time and cost are a greater advantage to the cross-sectional approach than the stability of behavioural characteristics over time.
In order to overcome the weaknesses of both the cross-sectional and longitudinal approach most developmental psychologists use a combination of the two in a single study as illustrated by Schaie (1965). He conducted a study of dependency behaviour in two, five and eight year olds. The first part of the study, which was cross-sectional, required data to be collected at all three age points. The longitudinal approach was then used following an interim of three years, when the two year olds reached the age of five and the five year olds reached the age of eight. If this study was simply longitudinal it would have taken six years before data collection was complete, therefore the combination of both approaches represents a fifty percent saving in time. It also allows for a check to be carried out on a possible cohort effect.
Both data collection methods, the longitudinal and cross-sectional approach, can produce different findings on the same study. Weschsler’s cross-sectional study showed a decline in intelligence from the age of twenty. In contrast Schaie and Willis’ longitudinal study of intelligence showed an increase in intelligence up until the age of fifty. The reasons for the different findings is firstly the cohort effect, different past histories, connected to the cross-sectional approach and secondly participant attrition, that is to say participant drop out, associated with the longitudinal approach.
In conclusion the longitudinal design is not the preferred approach for developmental psychologists because of the costs, time involved and participant attrition. On the other hand the cross-sectional design is very popular and is the preferred method as data covering a wide range of age points can be collected in a very short period of time. It is also cost effective and developmental questions can be answered quickly and efficiently.
References
Davey, G., (Ed) (2004) Complete Psychology. Oxon: Hodder & Stroughton.
Gleitman, H, Fridlund, A. J. F. & Reisberg, D. (2004) Psychology (6th Ed.). New York: W.W.Norton.
Cottrell, S., (2003) The Study Skills Handbook. (2nd Ed). Palgrave Macmillan.
Millar, W.S. (2004) Methods used by Developmental Psychologists. Transcript
PY 102C Developmental Psychology.
Student No: 03037041
PY1002C Developmental Psychology 1
