This function returns a value (or vector of values) of the bilateral Tornqvist price index.
tornqvist(data, start, end, interval = FALSE)The function returns a value (or vector of values) of the bilateral Tornqvist price index depending on the interval parameter. If the interval parameter is set to TRUE, the function returns a vector of price index values without dates. To get information about both price index values and corresponding dates, please see functions: price_indices or final_index. The function does not take into account aggregating over outlets or product subgroups (to consider these types of aggregating, please use the final_index function).
The user's data frame with information about sold products. It must contain columns: time (as Date in format: year-month-day,e.g. '2020-12-01'), prices (as positive numeric), quantities (as positive numeric) and prodID (as numeric, factor or character).
The base period (as character) limited to the year and month, e.g. "2020-03".
The research period (as character) limited to the year and month, e.g. "2020-04".
A logical value indicating whether the function is to compare the research period defined by end to the base period defined by start (then interval is set to FALSE) or all fixed base indices are to be calculated. In this latter case, all months from the time interval <start,end> are considered and start defines the base period (interval is set to TRUE).
Tornqvist, L. (1936). The Bank of Finland's Consumption Price Index. Bank of Finland Monthly Bulletin 10, 1-8.
(2004). Consumer Price Index Manual. Theory and practice. ILO/IMF/OECD/UNECE/Eurostat/The World Bank, International Labour Office (ILO), Geneva.
tornqvist(sugar, start="2018-12", end="2019-12")
tornqvist(milk, start="2018-12", end="2020-01", interval=TRUE)
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