Quantitative research begins with the collection of statistics, based on real data, observations or questionnaires. In the field of health, for example, researchers might measure and study the relationship between dietary intake and measurable physiological effects such as weight loss. Opinion surveys are a form of quantitative research in which respondents are asked a set of fixed questions and their responses are tallied. In the field of climate science, researchers compile and compare statistics such as temperature or atmospheric concentrations of carbon dioxide.
Sometimes quantitative research uses proxies as stand-ins for other quantities that cannot be directly measured. Tree-ring width, for example, is considered a reliable proxy of ambient environmental conditions such as the warmth of growing seasons or amount of rainfall. Although scientists cannot directly measure the temperature of past years, tree-ring width and other climate proxies have been used to provide a semi-quantitative record of average temperature in the Northern Hemisphere back to 1000 A.D.
Quantitative research is often contrasted with qualitative research, which is the non-numerical examination and interpretation of observations for the purpose of discovering underlying meanings and patterns of relationships. Qualitative research is generally considered to be exploratory and inductive in nature. It is used to get a general sense of what is happening and to form theories that can then be tested using quantitative research, which is viewed as confirmatory and deductive in nature.