epical allows you to calculate back and forth between epidemiological weeks and calendar dates.
Epidemiological weeks, also called "epi-weeks", "epi weeks", or "MMWR weeks" are a standardized method of assigning days to a week of the year. Epidemiological weeks are used by the WHO, CDC, and many other health organizations.
The WHO defines epidemiological weeks as starting on a Monday and ending on a Sunday. The CDC defines epidemiological weeks as starting on a Sunday and ending on a Saturday. In either case, the end of the first epidemiological week of the year by definition must fall at least four days into the year. In other words, the first epidemiological week always begins on a date between December 29 and January 4.
Every date can be assigned to a 7-day-long epidemiological week. Most years have 52 epidemiological weeks, but some have 53.
Because some dates in epidemiological week 1 can fall within the previous calendar year, and dates in epidemiological week 52/53 can fall within the subsequent calendar year, it is necessary to specify both the epidemiological week number and an "epidemiological year" so that dates can be grouped correctly by year.
To determine which epi-week a calendar date falls in, use
To add columns for epi-week and epi-year to a data frame with a date column,
To determine the start date of an epi-week or epi-year, use
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