There are two kinds of data sources: primary and secondary. Primary data are original data collected first-hand for a specific research purpose or project. Data which has been collected beforehand, either by someone else and/or for another purpose, is what is referred to as a secondary data source. Primary data collection, thru for example surveys, interviews, field observations or experiments, can be expensive and time consuming. For some purposes, secondary data collection might therefore be an option.
If you decide to use data from secondary data sources, make sure that you only collect data from trusted sources. An examples of a trusted sources is Statistics Sweden (Statistiska Centralbyrån, SCB), which is the government agency responsible for official statistics. Other trusted sources that provide data are the Environmental Protection Agency (Naturvårdsverket), the Swedish Meteorological and Hydrological Institute (SMHI) and the Swedish Board of Agriculture (Jordbruksverket).
There is usually no peer-reviewing of data, unless they are part of a scientific publication. When there is no formal quality control, you will need to determine for yourself if they are credible and relevant. You can find advice on what to consider when evaluating a source in the section Evaluating informal sources.