Pi-Change: Oil Price Application

This vignette analyzes absolute daily changes in WTI spot oil prices from 2000 through 2009. The prior locations correspond to events used in the original application script: rising prices around 2003, Hurricane Katrina, Iran sanctions, and the 2008 global recession. This example illustrates how PI-Change can use geopolitical and economic event times as soft prior support for structural changes in the observed series.

library(PiChange)
library(ggplot2)

data("wti_oil")

event_dates <- as.Date(c("2003-01-01", "2005-08-26", "2006-12-23", "2008-07-01"))
penalty <- construct_penalty(
  time = wti_oil$date,
  centers = event_dates,
  width = 26 * 5,
  method = "mbic",
  family = "zag"
)
fit <- pi_change(wti_oil$abs_price_change, penalty, min_seg_len = 13 * 5)

change_indices <- changepoints(fit)
wti_oil[change_indices, c("date", "price", "abs_price_change")]
#>            date price abs_price_change
#> 1093 2004-05-17 41.53             0.11
#> 1954 2007-10-23 86.45             1.15
#> 2271 2009-01-27 41.67             4.83

plot(
  fit,
  ylab = "Absolute daily price change",
  title = "WTI oil price volatility with PI-Change estimates"
)

Here width = 26 * 5 represents 130 observation positions, approximately six months of trading days, while min_seg_len = 13 * 5 requires approximately one quarter of trading days per segment. These are application-specific choices, not universal defaults. The original dates, matched observation indices, and penalty values are retained in fit$penalty.