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A longitudinal study (or longitudinal survey, or panel study) is a research design that involves repeated observations of the same variables (e.g., people) over long periods of time (i.e., uses longitudinal data). It is often a type of observational study, although it can also be structured as longitudinal randomized experiment. [1]
The study started with over 18,000 nationally representative individuals. [11] It involved more than 9,000 individuals as of 2009. Socio-Economic Panel, a longitudinal panel dataset of the population in Germany. It is a household-based study that started in 1984 and which reinterviews adult household members annually.
A popular repeated-measures design is the crossover study. A crossover study is a longitudinal study in which subjects receive a sequence of different treatments (or exposures). While crossover studies can be observational studies, many important crossover studies are controlled experiments.
Both datasets above are structured in the long format, which is where one row holds one observation per time. Another way to structure panel data would be the wide format where one row represents one observational unit for all points in time (for the example, the wide format would have only two (first example) or three (second example) rows of ...
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The PDF format is widely accepted and is considered the de facto standard for printable documents on the web. This means that users do not require the any proprietary plug-in to read geospatial PDFs created following the PDF 1.7 specification, which was published as ISO 32000-1 standard. [3]
The international standard convention —that east is positive—is consistent with a right-handed Cartesian coordinate system, with the North Pole up. A specific longitude may then be combined with a specific latitude (positive in the northern hemisphere ) to give a precise position on the Earth's surface.
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