<Ol> <Li> Studies that compare students who were retained against students who were considered for retention but were eventually promoted: Although these studies conclude that social promotion is beneficial, they are methodologically unsound due to selection bias in the group - allocation phase, in which the students selected for promotion were better (or less weak) and were indeed selected for promotion because the schools believed them to be stronger or more personally mature students whereas the students selected for promotion were weaker and selected because they were weaker . </Li> <Li> Studies which compare retained students to their own prior performance . These studies favor grade retention . However, these studies are biased because they do not adequately control for personal growth or changes in environmental factors (such as poverty). </Li> <Li> Studies which randomly assign a large pool of borderline students to promotion or retention . This style of research is methodologically sound and, if performed on a sufficiently large scale and with sufficiently detailed information collected, would provide valuable or even definitive information . However, schools and parents are unwilling to have a child's future affected by a random assignment, and so both due to institutional opposition and for ethical reasons these studies are simply not done . </Li> </Ol> <Li> Studies that compare students who were retained against students who were considered for retention but were eventually promoted: Although these studies conclude that social promotion is beneficial, they are methodologically unsound due to selection bias in the group - allocation phase, in which the students selected for promotion were better (or less weak) and were indeed selected for promotion because the schools believed them to be stronger or more personally mature students whereas the students selected for promotion were weaker and selected because they were weaker . </Li> <Li> Studies which compare retained students to their own prior performance . These studies favor grade retention . However, these studies are biased because they do not adequately control for personal growth or changes in environmental factors (such as poverty). </Li> <Li> Studies which randomly assign a large pool of borderline students to promotion or retention . This style of research is methodologically sound and, if performed on a sufficiently large scale and with sufficiently detailed information collected, would provide valuable or even definitive information . However, schools and parents are unwilling to have a child's future affected by a random assignment, and so both due to institutional opposition and for ethical reasons these studies are simply not done . </Li>

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