Kumar-2011-Research-Methodology_-A-Step-by-Step-Guide-for-Beginners

110
Research Methodology
an attitude in response to a questionnaire or interview has caused them to think about and
alter their attitude at the time of the post-test. This type of effect is known as the regression
effect.
The longitudinal study design
The before-and-after study design is appropriate for measuring the extent of change in a phenomenon,
situation, problem, attitude, and so on, but is less helpful for studying the pattern of
change. To determine the pattern of change in relation to time, a longitudinal design is used;
for example, when you wish to study the proportion of people adopting a programme over a
period. Longitudinal studies are also useful when you need to collect factual information on
a continuing basis. You may want to ascertain the trends in the demand for labour, immigration,
changes in the incidence of a disease or in the mortality, morbidity and fertility patterns
of a population.
In longitudinal studies the study population is visited a number of times at regular intervals,
usually over a long period, to collect the required information (see Figure 8.4). These intervals
are not fixed so their length may vary from study to study. Intervals might be as short as
a week or longer than a year. Irrespective of the size of the interval, the type of information
gathered each time is identical. Although the data collected is from the same study population,
it may or may not be from the same respondents. A longitudinal study can be seen as a series
of repetitive cross-sectional studies.
Study
population
Study
population
Study
population
Study
population
t t t t
= Data collection
t = Interval between data collection
Figure 8.4 The longitudinal study design
Longitudinal studies have many of the same disadvantages as before-and-after studies, in
some instances to an even greater degree. In addition, longitudinal studies can suffer from the
conditioning effect. This describes a situation where, if the same respondents are contacted
frequently, they begin to know what is expected of them and may respond to questions without
thought, or they may lose interest in the enquiry, with the same result.
The main advantage of a longitudinal study is that it allows the researcher to measure the
pattern of change and obtain factual information, requiring collection on a regular or
continuing basis, thus enhancing its accuracy.