Demography is the statistical study of human populations and subpopulations. Factors like aging, death, migration, birth and others are studied within this field. Vital statistics are used and this method is called the direct data method approach. This works well in the United States and Europe where statistics are routinely gathered. In other countries, the vital statistics may be incomplete. Indirect methods are used in countries where the data is not available or is incomplete. This can be seen in developing nations. One such method is the sister method, where women are asked how many of their sisters died, had children, how many, what age are they, etc. In most developed nations, population growth, which is one factor looked at in demographics, has diminished close to zero. In developed countries, people tend to have fewer offspring, although their life expectancies have risen. These are but a few examples of the statistics gathered in demography.
Demography is the study of human population trends, and in particular how these trends change over time and in response to various conditions such as disease, war, birth rates, distribution, migration, and so on. Demography can thus have a number of applications, and though the study of demography is typically historical in nature, it is relevant to political science, health and medicine, sociology, and other disciplines. The study of the ramifications of changes in population is known as social demography, though the statistics of demography are a field in and of themselves. Demographers generally rely on statistical information gleaned from surveys, the census, and polling data, and they are primarily interested in births, deaths, and life expectancy.