Including the omitted data would have changed the overall conclusions. The authors warn against citing and using research that has methodological flaws. School Uniform Relation to Attendance

A school uniform policy has been strongly suggested as a way to curb school violence and gang activity as way of dress is a common distinguishing characteristic of gang members. This sometimes has very tragic consequences. The uniformity in dress is thought to act as an equalizer and reduce the competition and its distracting effects especially for minority children (Lopez, 2003). The data on effect of school uniforms on student behavior is also not conclusive. The Long Beach Unified School District was one of the early initiatives in school uniforms in public schools. In the early 1990s because of increasing ethnic rivalry the District piloted a uniform policy in one school. By 1993-1994 there was enough support from the parents and community members to launch a uniform policy in ten additional elementary and middle schools. In their early evaluation they reported improvements in attendance, academic achievement, school safety and reduced ethnic and racial tensions. In the 1994-1995 school year all schools were required to determine appropriate uniforms, create a program for financial assistance and create compliance measures to allow for buttons and armbands for rights of freedom of expression and to include faith-based beliefs. As there was some resistance from some stakeholders on the grounds that it violated students' rights, there was a waiver policy included.

The Baltimore Cherry Hill Elementary School principal had also reported increased attendance, reduced suspensions and less frequent fighting among the results of the policy. In addition, other school districts have reported positive effects of a uniform school policy on gang violence, among them the Chicago, Alabama, and Houston (Konheim-Kalstein, 2006). However the Miami-Dade County reported a doubling in fights in uniformed schools. As pointed out by the author, in many of these school districts there were other changes in policy...

The research therefore is not conclusive but the testimonies of parents and school administrators are strong.
In order to address a methodological issue in the research Yeung (2009) included in his study the "value-added" estimation to mitigate potential bias that may result from omitted variables. Parental involvement can act as an unknown variable as a possible unobserved characteristic correlated with achievement which can also be correlated with the selection of school with a uniform is parental involvement. The study also uses two data sources, The Early Childhood Longitudinal Study- Kindergarten class and the National Educational Longitudinal Study (NELS). Despite these precautions the study found only one significant difference which can possibly be attributed to the school uniform policy - second grade math.

Research on the impact of school uniforms on both academic achievement and safety in schools is not yet conclusive enough. It is difficult to single out the wearing of a uniform as a contributing factor to attitudes, behavior and performance in schools. The inter-relatedness of the uniform with so many other factors such as parental involvement, home environment and peer relations in school needs to be investigated for conclusions to be more reliable.