In order to verify the statistical significance of these descriptive results, we computed effect
sizes (Cohen’s d) using means and standard deviations from the four datasets.10 When employing
this statistic, a positive value indicates enhancement of instructional leadership practice among
Thailand’s secondary school principals over time. A negative value represents a weakening in the
perceived pattern of practice between the two time periods.
In order to conduct this analysis, we set the Poovatanikul (1993) study as a reference group for
comparison with the 2008 data. As illustrated in Figure 5, there was no observed change in principal
instructional leadership from the 1990s to 2008 despite the implementation of the nation’s education
reforms. Indeed, the effect size estimates actually suggest some deterioration in ratings on defining a
school mission and managing the instructional program. Only developing a positive school learning