Article level metrics involve measurements for specific publications at the article level, as opposed to at the journal level or at the author level. Because they are article level metrics they involve sources which track and list articles, such as Google Scholar, as well as indexes which cite and index articles, including Scopus and other scholarly literature databases. Those are all traditional sources which use citation activity as the measurement to follow, so all of the sources are variations on a theme of citation counting. Other sources of article level metrics include alternative metric sources, which for the most part are Impact Story, Plum Analytics and Altmetric. These alternative sources use data other than citation activity to measure and track importance. Any thorough article level metric search should include both traditional and alternative sources.
Traditional article metric sources are citation based - they track how articles cite each other and are all variations on a theme of citation counting. Therefore they are slow to accumulate and build over time. New articles will generally have lower citation rates than older articles, although many articles are never cited by others at all. The sources for traditional metrics are generally article indexes or databases, like Scopus, or article listings like Google Scholar.
Alternative metrics or "altmetrics" come from many sources, some of which are scholarly, many of which are not. All kinds of article uses are tracked, from downloads to tweets to comments to news stories. They often accumulate quickly, are frequently social in nature, and are still developing as a source of metrics information.
The most common types of altmetric information comes from:
The three sources listed below are the primary sources of alternative metrics, currently.