RESEARCH METHOD COHEN ok

296 EXPERIMENTS AND META-ANALYSIS
Box 13.6
Class size and learning in well-controlled and poorly controlled studies
Achievement in percentile ranks
90
80
70
60
50
Well-controlled studies
Poorly controlled studies
10 20 30 40
Class size
Regression lines for the regression of achievement (expressed in percentile ranks) onto class-size
for studies that were well-controlled and poorly controlled in the assignment of pupils to classes.
Source:adaptedfromGlassandSmith1978
When the researchers derived similar curves for
avarietyofcircumstancesthattheyhypothesized
would alter the basic relationship (for example,
grade level, subject taught, pupil ability etc.),
virtually none of these special circumstances
altered the basic relationship. Only one factor
substantially affected the curve – whether the
original study controlled adequately in the
experimental sense for initial differences among
pupils and teachers in smaller and larger classes.
Adequate and inadequate control curves are set
out in Box 13.6. 4