In the early years of the Chesapeake Bay settlements, colonial officials found it difficult to attract and retain laborers under the harsh frontier conditions, and there was a high mortality rate.[4] Most laborers came from Britain as indentured servants, who signed contracts of indenture to pay with work for their passage, their upkeep and training, usually on a farm, as the colonies were based in agriculture. These indentured servants were young people who intended to become permanent residents. Some masters treated them as well or as poorly as family members. In some cases, convicted criminals were transported to the colonies as indentured servants, rather than being imprisoned. The indentured servants were not slaves. The planters found that the major problem with indentured servants was that many left after several years, just when they had become skilled and the most valuable workers. In addition, an improving economy in England in the late 17th and early 18th centuries meant that fewer workers chose to go to the colonies. Historians estimate that more than half of all white immigrants to the English colonies of North America during the 17th and 18th centuries came as indentured servants. The number of indentured servants among immigrants was particularly high in the South. Many Germans, Scots-Irish, and Irish came to the colonies in the 18th century, settling in the backcountry of Pennsylvania and further south.[4]