The impact factor was first developed for comparing journals. It is still the oldest and most used indicator for measuring the impact of journals. Clarivate Analytics has the exclusive rights to the impact factor and the IF-numbers are only available in the Web of Knowledge Journal Citation Reports (JCR) database, which is updated yearly.
There are many restrictions when it comes to the use of the impact factor, and the indicators have been criticised e.g. for the following reasons: