Compute expenditure shares for each product and time period
shares(x, pvar, qvar, pervar, prodID)
an n by p data frame of expenditure shares
A dataframe containing price, quantity, a time period identifier and a product identifier. It must have column names.
A character string for the name of the price variable
A character string for the name of the quantity variable. For elementary indexes a quantity variable is not required for the calculations and you must specify qvar = "".
A character string for the name of the time variable. This variable must contain integers starting at period 1 and increasing in increments of 1 period. There may be observations on multiple products for each time period.
A character string for the name of the product identifier