Opponents argue that enforcement of no-nit policies has not significantly reduced head lice incidence in school settings, and that the risks and disadvantages of the no-nit policies outweigh their associated benefits. As is the case in assessing school screening efforts (see Screening above), assessing the effectiveness of no-nit policies is complicated by the sparseness of head lice prevalence data. To the extent that no-nit policies increase the efficiency of school screening programs (see Benefits of nit-free population above), no-nit policies may lead to more rapid containment of head lice outbreaks. However, , the correlation between no-nit policies and head lice incidence has not been examined. The no nit policy involves a number of issues, such as the circumstances under which children will be inspected, the criteria to infer infestation, the procedures for dealing with infested children upon discovery, and the criteria to permit previously infested children to return to school. These issues are controversial. In particular, a number of health researchers and organizations object to the required removal of nits. Opponents to the no-nit policy point out that nits, being empty eggshells, have no clinical importance. Time-consuming nit removal, therefore, has no direct effect on transmission. This has led to the perception that the no-nit policy serves only to ease the workload of school nurses. Proponents of the no-nit policy counter that only a consistently nit-free child can be reliably shown to be infestation-free. That is, the presence of nits serves as a proxy for infestation status. Proponents argue that such a proxy is necessary because lice screening is prone to false negative conclusions (i.e., failure to find lice present on actively infested children). Contributing to the controversial nature of school head lice policies is a lack of available data on their importance and efficacy. For example, , the transmissibility of head lice in school settings is largely unknown. Further, control-matched studies of the effect of specific school policies on head lice incidence are also lacking. Schools in developed countries such as the United States, Canada, and Australia commonly exclude infested students, and prevent return of those students until all lice, eggs, and nits are removed. Policies in the United Kingdom extend to legal action against parents of infested students although the NHS does not recommend exclusion from school. Attitudes in developing countries are often quite different. Many societies consider head louse infestation to be normal, if even remarked upon at all.