Contributions

In piar: Price Index Aggregation

Product contributions

It's often convenient to decompose an index into the (additive) contribution of each price relative, also known as the percent-change contribution. This can be done with the same work flow used in vignette("piar"), specifying contrib = TRUE when calling elemental_index().

library(piar)

# Make an aggregation structure.
ms_weights[c("level1", "level2")] <- 
  expand_classification(ms_weights$classification)

pias <- ms_weights[c("level1", "level2", "business", "weight")] |>
  as_aggregation_structure()

# Make elemental index with contributions.
elementals <- ms_prices |>
  transform(
    relative = price_relative(price, period = period, product = product)
  ) |>
  elemental_index(
    relative ~ period + business,
    product = product,
    na.rm = TRUE,
    contrib = TRUE
  )

As with index values, percent-change contributions for a given level of the index can be extracted as a matrix.

contrib(elementals, level = "B1")

Or as a data frame.

contrib2DF(elementals, level = "B1")

Aggregating the elemental indexes automatically aggregates percent-change contributions, so no extra steps are needed after the elemental indexes are made.

index <- aggregate(elementals, pias, na.rm = TRUE)

contrib(index)

Index contributions

After an index has been calculated, it's often useful to compute the contribution of higher-level indexes towards the total index. The easiest way to do this with a collection of pre-computed index values is to simply coerce them into an index object with the index values as contributions and reaggregate with a restricted aggregation structure.

index <- as_index(as.matrix(index), contrib = TRUE)

If the index values are already an index object, it's also possible to directly replace the contributions with the set_contrib_from_index() function. We can now cut the aggregation structure to keep only the top two levels and reaggregate to get the contribution of the second-level indexes to the top level index.

set_contrib_from_index(index) |>
  aggregate(cut(pias, 2)) |>
  contrib()

The same approach works with a fixed-base index as well.

chain(index) |>
  set_contrib_from_index() |>
  aggregate(cut(pias, 2)) |>
  contrib()

piar documentation built on April 3, 2025, 7:38 p.m.