In a longitudinal study, researchers repeatedly examine the same individuals to detect any changes that might occur over a period of time. Show
Longitudinal studies are a type of correlational research in which researchers observe and collect data on a number of variables without trying to influence those variables. While they are most commonly used in medicine, economics, and epidemiology, longitudinal studies can also be found in the other social or medical sciences. How long is a longitudinal study?No set amount of time is required for a longitudinal study, so long as the participants are repeatedly observed. They can range from as short as a few weeks to as long as several decades. However, they usually last at least a year, oftentimes several. One of the longest longitudinal studies, the Harvard Study of Adult Development, has been collecting data on the physical and mental health of a group of Boston men for over 80 years! Longitudinal vs cross-sectional studiesThe opposite of a longitudinal study is a cross-sectional study. While longitudinal studies repeatedly observe the same participants over a period of time, cross-sectional studies examine different samples (or a “cross-section”) of the population at one point in time. They can be used to provide a snapshot of a group or society at a specific moment.  Both types of study can prove useful in research. Because cross-sectional studies are shorter and therefore cheaper to carry out, they can be used to discover correlations that can then be investigated in a longitudinal study. Cross-sectional vs longitudinal exampleYou want to study the relationship between smoking and stomach cancer. You first conduct a cross-sectional study to see if there is a link between smoking and stomach cancer, and you discover that a link exists in men but not in women.You then decide to design a longitudinal study to further examine this relationship in men. Without the cross-sectional study first, you would not have known to focus on men in particular. How to perform a longitudinal studyIf you want to implement a longitudinal study, you have two choices: collecting your own data or using data already gathered by somebody else. Using data from other sourcesMany governments or research centers carry out longitudinal studies and make the data freely available to the general public. For example, anyone can access data from the 1970 British Cohort Study, which has followed the lives of 17,000 Brits since their births in a single week in 1970, through the UK Data Service website. These statistics are generally very trustworthy and allow you to investigate changes over a long period of time. However, they are more restrictive than data you collect yourself. To preserve the anonymity of the participants, the data collected is often aggregated so that it can only be analyzed on a regional level. You will also be restricted to whichever variables the original researchers decided to investigate. If you choose to go this route, you should carefully examine the source of the dataset as well as what data is available to you. Collecting your own dataIf you choose to collect your own data, the way you go about it will be determined by the type of longitudinal study you choose to perform. You can choose to conduct a retrospective or a prospective study.
Retrospective studies are generally less expensive and take less time than prospective studies, but are more prone to measurement error. Retrospective vs prospective exampleIn a retrospective study, you might look at past medical records of patients to see whether those who developed this cancer had previously smoked. In a prospective study, you might follow a group of both smokers and non-smokers over time to see if they develop cancer later on.Advantages and disadvantages of longitudinal studiesLike any other research design, longitudinal studies have their tradeoffs: they provide a unique set of benefits, but also come with some downsides. Longitudinal studies allow researchers to follow their subjects in real time. This means you can better establish the real sequence of events, allowing you insight into cause-and-effect relationships. ExampleA cross-sectional study on the impact of police on crime might find that more police are associated with greater crime and wrongly conclude that police cause crime when it is the other way around. However, a longitudinal study would be able to observe the rise or fall in crime some time after increasing the number of police in an area.Longitudinal studies also allow repeated observations of the same individual over time. This means any changes in the outcome variable cannot be attributed to differences between individuals. ExampleYou decide to study how a particular weight-training program affects athletic performance. If you choose a longitudinal study, the impact of natural talent on performance should be eliminated, since that would not change over the study period.Prospective longitudinal studies eliminate the risk of recall bias, or the inability to correctly recall past events. ExampleYou are studying the effect of low-carb diets on weight loss. If you asked your subjects to remember how many carbs or how much they weighed at any point in time in the past, they might have difficulty doing so. In a longitudinal study, you can keep track of these variables in real time.Longitudinal studies are time-consuming and often more expensive than other types of studies, so they require significant commitment and resources to be effective. Since longitudinal studies repeatedly observe subjects over a period of time, any potential insights from the study can take a while to be discovered. ExampleIn the study examining the links between smoking and stomach cancer, you have to wait several years to see any results since the negative effects of smoking accumulate over decades.Attrition, which occurs when participants drop out of a study, is common in longitudinal studies and may result in invalid conclusions. ExampleIn your study on the impact of low-carb diets on weight loss, participants who are not seeing much success might feel more discouraged and thus more likely to drop out. The diet might therefore appear to be more successful than it actually is!Frequently asked questions about longitudinal studies
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In Paper 2 Clinical, it is important that you make the methods relate to clinical examples. You need to think carefully about why these methods are particularly useful when doing research in clinical psychology. Before we get into this, let’s remind ourselves of what is meant by cross-sectional studies. Describe what is meant by a cross sectional study (3)
Example of CS study in clinical psychology: Hyde et al (1994): Cognitive decline and schizophrenia: Patients with chronic schizophrenia, aged from 18 to 69 years were divided into five cohorts: 18-29, 30-39, 40-49, 50-59 and 60-69 (five levels of the IV) and intellectual deterioration was tested using the Mini-Mental State Examination and the Dementia Rating Scale plus other tests known to be sensitive to cognitive impairment in progressive dementia (the DVs). The Pps had passed a rigorous screen for any co-morbid condition that might be affecting their CNS function. There was no evidence of accelerated intellectual decline. Performance on the Boston Naming Test significantly declined with age, this was mainly due to age rather than duration of illness. Mean test performances were abnormal across all age groups but intellectual function does not seem to markedly decline during the adulthood of patients with schizophrenia. Describe what is meant by a longitudinal study (3)
Activity: Think about Hyde’s cross-sectional study; how would cognitive decline in schizophrenia be assessed in a longitudinal study? What problems would Hyde et al have encountered if they had done the study longitudinally? What advantages might this approach have had? Example of a longitudinal study on the same topic, cognitive decline in people with schizophrenia: Harvey et al (1999): Cognitive decline in late-life schizophrenia: a longitudinal study of geriatric chronically hospitalized patients.
The study looked at 326 geriatric, chronic, long-stay schizophrenic patients. The patients showed cognitive and functional impairments that were in decline, post illness-onset. The study lasted 30 months and there were two separate assessments of the patients. Cognitive and functional impairments (the DV) was measured using the Clinical Dementia Rating (CDR). Of those Pps who had less severe scores at the start of the study (baseline), 30% had declined further by the second assessment and only 7% of the sample with lower scores at baseline appeared to improve in their functioning. Several characteristics of the patients at baseline assessment predicted increased risk for cognitive and functional decline, including lower levels of education, older age, and more severe positive symptoms. Activity: Compare Harvey et al and Hyde et al; what strengths are there in Harvey’s study which are not present in that of Hyde and vice versa? You need to be able to compare cross sectional and longitudinal studies. Make sure you have practiced this! Always use examples such as Hyde et al and Harvey et al in your writing. Technique tips: When you see the word compare, think: similarities and differences. State a point about one of the things you are asked to compare and then say how the other is similar in one way but different in another. Do this as many times as you can in the time allowed. Remember the ‘Venn Diagram’ can be a helpful way to plan your answer. Content tips:
Useful connectives: whereas, in contrast, on the other hand, similarly, but, yet, alternatively, nevertheless, both A very useful concept for evaluation: Cohort EffectsThe cohort are the group of people being studied. Cohort effects refers to factors relating to this group which might make them special in some way and thus not entirely comparable to another similar group who might be selected in years to come in a replication of the study. These factors cannot be controlled and therefore could become a confounding variable. Classic cohort effects relate to things which happened only to that specific generation of children, e.g.
When you conduct a cross sectional study, each of the group of children may have lived through differing social and economic circumstances and these will not necessarily be the same meaning that you cannot be sure that any differences between the children’s performance/behaviour is due to differences in their age or differing experiences of each cohort. This is a problem for cross sectional studies. In longitudinal studies cohort effects can affect the generalisability of the results. It is possible that the finding and conclusions will only apply to groups of people who have lived through similar social and economic circumstances to the original group of participants. cross-and-long-models: A handout containing the text above. Make your own top trumps cards:
how-science-research-methods3 psychodcross-sectional-and-longitudinal In Paper 3, you might be asked about cross sectional and longitudinal studies in general. Of course you may use your examples from clinical to help illustrate your answers however, it is also useful to practice some questions with non-clinical examples. Practice Questions (a) Seif is an educational psychologist who is interested in researching whether students perform better if they have one examination at the end of a two-year course or if students perform better if they have several examinations throughout the two years of the course. Seif has decided to carry out a longitudinal study to compare the two approaches to student examinations. Describe how Seif may carry out his longitudinal study. (7) You may wish to include some of the following:
(b) Evaluate the longitudinal research method, as it is used in psychology. (4) Charles is carrying out an experiment on young children. He wants to see if there is a difference between 4 year olds and 8 year olds in their ability to understand that a tall, narrow glass of water can hold the same amount of water as a short, wide glass. Charles had two short, wide glasses that held the same amount of water. The children watched him pour one of these glasses of water into a tall, narrow glass. Charles then asked the children if the short, wide glass and the tall, narrow glass had the same amount of water in them. (a) Identify the independent variable (IV) in this experiment. (1) (b) Identify the dependent variable (DV) in this experiment. (1) (c) (i) Identify the experimental design Charles used in this study. (1) (ii) Outline one strength and one weakness of the experimental design you identified in 12(c)(i). (4) (d) Describe the ethical issues Charles must consider when carrying out his experiment with the children. (4) |
What is a study in which the performance of the same group of participants is examined at different ages?
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