tend to classify different societies according to the degree to which different groups within a society have unequal access to advantages such as resources, prestige, or power. Virtually all societies have developed some degree of inequality among their people through the process of social stratification, the division of members of a society into levels with unequal wealth, prestige, or power. Sociologists place societies in three broad categories: pre-industrial, industrial, and postindustrial. Pre-industrial In a pre-industrial society, food production, which is carried out through the use of human and animal labor, is the main economic activity. These societies can be subdivided according to their level of technology and their method of producing food. These subdivisions are hunting and gathering, pastoral, horticultural, and agricultural. Hunting and gathering The main form of food production in hunter-gatherer societies is the daily collection of wild plants and the hunting of wild animals. Hunter-gatherers move around constantly in search of food. As a result, they do not build permanent villages or create a wide variety of artifacts, and usually only form small groups such as bands and tribes. However, some hunting and gathering societies in areas with abundant resources (such as the people of Tlingit in North America) lived in larger groups and formed complex hierarchical social structures such as chiefdom. The need for mobility also limits the size of these societies. Bands consist of 15 to 50 people related by kinship. Statuses within the tribe are relatively equal, and decisions are reached through general agreement. The ties that bind the tribe are more complex than those of the bands. Leadership is personal—charismatic—and used for special purposes only in tribal society. There are no political offices containing real power, and a chief is merely a person of influence. The family forms the main social unit, with most members being related by birth or marriage. The anthropologist Marshall Sahlins described hunter-gatherers as the "original affluent society" due to their extended leisure time: adults in foraging and horticultural societies work, on average, about 6.5 hours a day, whereas people in agricultural and industrial societies work on average 8.8 hours a day. Pastoral Pastoralism is a slightly more efficient form of subsistence. Rather than searching for food on a daily basis, members of a pastoral society rely on domesticated herd animals to meet their food needs. Pastoralists live a nomadic life, moving their herds from one pasture to another.