Derrington 2012 thesis.pdf - Anglia Ruskin Research Online

Effort was made to match the subject teachers for each student but this was not
possible in practical terms due to the large number of teachers involved. The
assumption was that all teachers and instructors were equally qualified to assess the
students and meant that, whether the assessments were done well or poorly, every
student had an equal chance to be assessed accurately and any anomaly in the
responses would be cancelled out. This assumption could be made because individual
differences in scores would not count for much. The assumption had to be made
because it was not just three teachers who were involved or even the same three
teachers for each student at every point of data collection, mostly due to changes in
the staff team over the project’s duration.
5.6 Missing data imputation
The students were encouraged to complete all items but when this was not possible
the response was estimated by the mean response value of the items answered by the
student on that inventory. Total raw scores could only be converted to T scores with
this imputation and this was according to the manual to the Beck Youth Inventories
second edition (Beck et al., 2005, p.12).
When there were some missing data in the teachers’ questionnaires, these following
rules were used:
1. When an individual question was missing at any point after baseline, the
standard method of last observation carried forward (Shao and Zhong, 2003)
was used. Therefore the assumption was made that the response remains
constant at the last observed value.
2. For missing baseline data, the mean was used for that cohort. For example,
a student in group A from CVC with a missing baseline score for a particular
question received, as their score, the mean for that question from all group A
and CVC students.
3. There was no imputation of total scores. This meant that individual
questions were imputed but the scores were not added up for analysis.
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