Researchers usually cannot make direct observations of every individual in the population they are studying. Instead, they collect data from a subset of individuals – a sample – and use those observations to make inferences about the entire population. Ideally, the sample corresponds to the larger population on the characteristic(s) of interest. In that case, the researcher's conclusions from the sample are probably applicable to the entire population. This type of correspondence between the sample and the larger population is most important when a researcher wants to know what proportion of the population has a certain characteristic – like a particular opinion or a demographic feature. Public opinion polls that try to describe the percentage of the population that plans to vote for a particular candidate, for example, require a sample that is highly representative of the population. Probability samples and convenience samples Two general approaches to sampling are used in social science research. With probability sampling, all elements (e.g., persons, households) in the population have some opportunity of being included in the sample, and the mathematical probability that any one of them will be selected can be calculated. Because some members of the population have no chance of being sampled, the extent to which a convenience sample – regardless of its size – actually represents the entire population cannot be known. Recruiting a probability sample is not always a priority for researchers. A scientist can demonstrate that a particular trait occurs in a population by documenting a single instance. For example, the assertion that all lesbians are mentally ill can be refuted by documenting the existence of even one lesbian who is free from psychopathology. Another situation in which a probability sample is not necessary is when a researcher wishes to describe a particular group in an exploratory way. For example, interviewing 25 people with AIDS (PWAs) about their experiences with HIV could provide valuable insights about stress and coping, even though it would not yield data about the proportion of PWAs in the general population who share those experiences.