Research Impact generally refers to the effect research has in areas outside the academia - economy, society, culture, etc. Whilst it is challenging to comprehensively and accurately measure research impact in larger context, there are some quantitative measures based on citation metrics that are commonly used by the academia as an indicator of the influence for research and researchers.
These citation metrics are useful to researchers to help them identify key publications and key authors in a field. These metrics can also be used to track article impact, journal impact and the author’s impact.
Alternative Metrics or Altmetrics are increasingly used to capture and measure online sharing, mentions, views and downloads of scholarly works in social media such as Twitter, Facebook, Google+, blog post and social bookmarks, etc. Compared to the core citation metrics mentioned above, altmetrics is viewed as one of the ways to measure the immediate impact of a work, especially before citation based metrics are available in citation databases.
Citation metrics are calculated for the works indexed at the same database.
Citation metrics involve counting the number of times an article (in a database) is cited by other works (within the same database) to measure the impact of a publication or author.
The incompleteness of citation databases
Database vendors and publishers always try to include the most influential and important journals but they do not collect and index the world's entire publication of scholarly works. Many publications are not included in any of the citation databases, thus would not have citation counts available.
The content and year coverage of the databases also vary - one can cover more journals and articles, while another database cover less journal titles. Citation counts of the same article would then vary across different citation databases because of the difference of content coverage in each database.
Using citation metrics to measure research impact is just ONE quantitative way.
Citation metric is only ONE quantitative indicator of research performance; other quantitative and qualitative approaches to measure research impact should also be used collectively to obtain a holistic view of the research and its impact.