Back period

Offset a vector prices or quantities by computing the position one (or more) period backwards for each product.

Usage

back_period(period, product = NULL, match_first = TRUE, offset = 1L)

Arguments

period

[factor] A factor, or something that can be coerced into one, that gives the time period for each transaction. The ordering of time periods follows the levels of period to agree with cut().

product

[factor] A factor, or something that can be coerced into one, that gives the product identifier for each transaction. The default is to assume that all transactions are for the same product.

match_first

[logical(1)] Should products in the first period match with themselves (the default)?

offset

[integer(1)] The number of periods to offset. The default offsets by one period (back period). Setting to nlevels(period) gives the the base period.

Value

A numeric vector of indices giving the position of the the back periods.

Note

By definition, there must be at most one transaction for each product in each time period to determine a back period. If multiple transactions correspond to a period-product pair, then the back period at a point in time is always the first position for that product in the previous period.

See also

outliers() for common methods to detect outliers for price relatives.

rs_pairs in the rsmatrix package for making sales pairs.

Examples

prices <- data.frame(
  price = 1:6,
  product = factor(c("a", "b")),
  period = factor(c(1, 1, 2, 2, 3, 3))
)

with(prices, back_period(period, product))
#> [1] 1 2 1 2 3 4

# Make fixed-base price relatives.
with(
  prices,
  price / price[back_period(period, product, offset = nlevels(period))]
)
#> [1] 1 1 3 2 5 3

# Change the base period with relevel().
with(
  prices,
  price / price[
    back_period(relevel(period, "2"), product, offset = nlevels(period))
  ]
)
#> [1] 0.3333333 0.5000000 1.0000000 1.0000000 1.6666667 1.5000000