Housing quality is measured using objective and subjective indicators, as well as quality of life. On the one hand, subjective indicators investigate housing quality at the individual level; it is dependent on personal experience, so it is difficult to measure; for example, houses are more likely to be preferable than flats, central areas rather than peripheral areas, modern rather than traditional dwellings, and respectable rather than rough neighborhoods. In the twentieth century, a closer look at housing densities was thought to reduce densities in downtown areas on one hand and to increase densities in urban suburbs on the other. Measurement to explore the quality of life, two basic categories have been developed: subjective and objective. Skills, beliefs, and knowledge are examples of subjective indicators. They are referring to the residents' contentment and having a good impact. However, objective indicators are based on physical items that are easier to quantify, such as food, shelter, pollution levels, housing expenses, standard of living, money, and access to goods and services.