RESEARCH METHOD COHEN ok

214 SURVEYS AND DEVELOPMENTAL STUDIES
Box 9.2
Types of developmental research
TREND STUDY
Recorded
data
on factors
a, b, c
Predicted
patterns
Observations on a, b, c.
1 2 3 4 5 6 n
COHORT STUDY
Sample A
Sample A
Sample A
Sample A
Sample A
Observation
1
Observation
2
Observation
3
Observation
4
Observation
5
CROSS-SECTIONAL STUDY
Sample
A
Sample
B
Sample
C
Sample
D
Sample
E
Retrospective
SOCIAL PROCESSES
Prospective
OVER TIME
Strengths and weaknesses of longitudinal,
cohort and cross-sectional studies
Longitudinal studies of the cohort analysis type
have an important place in the research armoury
of educational investigators. Longitudinal studies
have considerable potential for yielding rich data
that can trace changes over time, and with
great accuracy (Gorard 2001: 86). On the other
hand, they suffer from problems of attrition
(participants leaving the research over time, a
particular problem in panel studies which research
the same individuals over time), and they can
be expensive to conduct in terms of time and
money (Ruspini, 2002: 71). Gorard (2001) reports
astudyofcareersandidentitiesthathadaninitial
response rate of between 60 and 70 per cent in the
first round, and then risked dropping to 25 per cent
by the third round, becoming increasingly more
middle class in each wave of the study. Gorard
(2001) also discusses a Youth Cohort Study in
which only 45 per cent of the respondents took part
in all three waves of the data collection. Ruspini
(2002: 72) identifies an attrition rate of 78 per cent
in the three waves of the European Community
Household Panel survey of the United Kingdom
in 1997.
Ruspini (2002) also indicates how a small
measurement error in a longitudinal study may
be compounded over time. She gives the example
of an error in income occurring at a point in
time that could lead to ‘false transitions’ appearing
over time in regard to poverty and unemployment
(Ruspini 2002: 72).
Further, long-term studies, Gorard (2001: 86)
avers, face ‘a threat to internal validity’ that
stems from the need ‘to test and retest the same
individuals’. Dooley (2001: 120) terms this ‘pretest
sensitisation’; it is also termed ‘panel conditioning’
or ‘time-in sample bias’ (Ruspini, 2002: 73). Here
the first interview in an interview survey can
cause changes in the second interview, i.e. the
first interview may set up a self-fulfilling prophecy
that is recorded in the second interview. Dooley