If 'X' is the whole range of article content posted in 2020 and 'Y' is the number of periods these content articles had been cited in indexed journals in the course of 2021 then, journal impact factor = Y/X.
The chart https://gerardeool465463.blognody.com/29552639/the-ultimate-guide-to-annals-of-clinical-and-medical-case-reports