 By ensuring that each block of units is sold to families from ethnicities roughly comparable to the national average, it seeks to avoid physical racial segregation and formation of ethnic enclaves common in other multi-racial societies.Partly in response to public sentiment against the alleged formation of "permanent resident enclaves", where some flats appeared dominated by permanent residents from a single nationality, the HDB introduced the Singapore Permanent Resident Quota which took effect on 5 March 2010.Permanent residents were subject to a cap of 5% of the households per block in every housing estate.