Journal-level metrics attempt to quantify a journal's impact by analyzing the citations arising from the articles it publishes.
Advantages: Journal metrics can give a sense of which journals are popular and/or respected within a specific field.
Disadvantages: These metrics effectively average the impact of a journal's articles and authors, so they hide variations among articles and authors. Journal metrics also are not generalizable across disciplines.
Author-level metrics attempt to quantify impact by analyzing citations arising from an individual author's publications.
Advantages: These metrics can give a more holistic idea of author impact by including a wide range of journals.
Disadvantages:These metrics are biased toward more prolific and more established authors. They also are not generalizable across disciplines.
The purpose of article-level metrics is to establish the impact of an article. The most common way of evaluating this is to count the number of times an article has been cited in other articles. Citation counts vary by the database used because the number of journal titles included varies.
The term altmetrics refers to new, alternative ways of assessing the impact of authors and publications. It presumes that value can be assessed by counting online shares, saves, reviews, adaptations and mentions. Altmetrics take into account a variety of research products, including gray literature; research blogs; data sets; citation managers, like Mendeley; and social media, such as Facebook and Twitter