sensitivity: Sensitivities of a Stressed Model
In spesenti/SWIM: Scenario Weights for Importance Measurement

Description Usage Arguments Details Value Author(s) References See Also Examples

Provides different sensitivity measures that compare the stressed and the baseline model.

sensitivity(
  object,
  xCol = "all",
  wCol = "all",
  type = c("Gamma", "Kolmogorov", "Wasserstein", "reverse", "all"),
  f = NULL,
  k = NULL,
  s = NULL,
  p = 1
)

`object`	A `SWIM` or `SWIMw` object.
`xCol`	Numeric or character vector, (names of) the columns of the underlying data of the `object` (`default = "all"`). If `xCol = NULL`, only the transformed data `f(x)` is considered.
`wCol`	Vector, the columns of the scenario weights of the `object` corresponding to different stresses (`default = "all"`).
`type`	Character, one of `"Gamma", "Kolmogorov", "Wasserstein", "reverse", "all"` (`default = "all"`).
`f`	A function, or list of functions, that, applied to `x`, constitute the transformation of the data for which the sensitivity is calculated.
`k`	A vector or list of vectors, same length as `f`, indicating which columns of `x` each function in `f` operates on. When `f` is a list, `k[[i]]` corresponds to the input variables of `f[[i]]`.
`s`	A function that, applied to `x`, defines the reverse sensitivity measure. If `type = "reverse"` and `s = NULL`, defaults to `type = "Gamma"`.
`p`	Numeric vector, the p-th moment of Wasserstein distance (`default = 1`).

Provides sensitivity measures that compare the stressed and the baseline model. Implemented sensitivity measures:

Gamma, the Reverse Sensitivity Measure, defined for a random variable Y and scenario weights w by

Gamma = ( E(Y * w) - E(Y) ) / c,

where c is a normalisation constant such that |Gamma| <= 1, see \insertCitePesenti2019reverseSWIM. Loosely speaking, the Reverse Sensitivity Measure is the normalised difference between the first moment of the stressed and the baseline distributions of Y.
Kolmogorov, the Kolmogorov distance, defined for distribution functions F,G by

Kolmogorov = sup |F(x) - G(x)|.
Wasserstein, the Wasserstein distance of order 1, defined for two distribution functions F,G by

Wasserstein = \int |F(x) - G(x)| dx.
reverse, the General Reverse Sensitivity Measure, defined for a random variable Y, scenario weights w, and a function s:R -> R by

epsilon = ( E(s(Y) * w) - E(s(Y)) ) / c,

where c is a normalisation constant such that |epsilon| <= 1. Gamma is a special instance of the reverse sensitivity measure when s is the identity function.

If f and k are provided, the sensitivity of the transformed data is returned.

A data.frame containing the sensitivity measures of the stressed model with rows corresponding to different random variables. The first two rows specify the stress and type of the sensitivity measure.

Silvana M. Pesenti, Zhuomin Mao

\insertRef

Pesenti2019reverseSWIM

See importance_rank for ranking of random variables according to their sensitivities, plot_sensitivity for plotting sensitivity measures and summary for summary statistics of a stressed model.

## example with a stress on VaR
set.seed(0)
x <- as.data.frame(cbind(
  "log-normal" = rlnorm(1000),
  "gamma" = rgamma(1000, shape = 2)))
res1 <- stress(type = "VaR", x = x,
  alpha = c(0.9, 0.95), q_ratio = 1.05)

sensitivity(res1, wCol = 1, type = "all")
## sensitivity of log-transformed data
sensitivity(res1, wCol = 1, type = "all",
  f = list(function(x)log(x), function(x)log(x)), k = list(1,2))

## Consider the portfolio Y = X1 + X2 + X3 + X4 + X5,
## where (X1, X2, X3, X4, X5) are correlated normally
## distributed with equal mean and different standard deviations,
## see the README for further details.


## Not run: 
set.seed(0)
SD <- c(70, 45, 50, 60, 75)
Corr <- matrix(rep(0.5, 5 ^ 2), nrow = 5) + diag(rep(1 - 0.5, 5))
if (!requireNamespace("mvtnorm", quietly = TRUE))
   stop("Package \"mvtnorm\" needed for this function
   to work. Please install it.")
x <- mvtnorm::rmvnorm(10 ^ 5,
   mean =  rep(100, 5),
   sigma = (SD %*% t(SD)) * Corr)
data <- data.frame(rowSums(x), x)
names(data) <- c("Y", "X1", "X2", "X3", "X4", "X5")
rev.stress <- stress(type = "VaR", x = data,
   alpha = c(0.75, 0.9), q_ratio = 1.1, k = 1)

sensitivity(rev.stress, type = "all")
## sensitivity to sub-portfolios X1 + X2 and X3 + X4
sensitivity(rev.stress, xCol = NULL, type = "Gamma",
  f = rep(list(function(x)x[1] + x[2]), 2), k = list(c(2, 3), c(4, 5)))
plot_sensitivity(rev.stress, xCol = 2:6, type = "Gamma")
importance_rank(rev.stress, xCol = 2:6, type = "Gamma")

## End(Not run)