bcajack2 {bcaboot}

Nonparametric bias-corrected and accelerated bootstrap confidence limits

Description

This function is a version of bcajack that allows all the recomputations of the original statistic function f to be carried out separately. This is an advantage if f is time-consuming, in which case the B replications for the nonparametric bca calculations might need to be done on a distributed basis.

To use bcajack2 in this mode, we first compute a list Blist via Blist <- list(Y = Y, tt = tt, t0 = t0). Here tt is a vector of length B having i-th entry tt[i] <- func(x[Ii,], ...), where x is the n \times p data matrix and Ii is a bootstrap vector of (observation) indices. Y is a B by n count matrix, whose i-th row is the counts corresponding to Ii. For example if n = 5 and ⁠Ii = (2, 5, 2, 1, 4)⁠, then ⁠Yi = (1, 2, 0, 1, 1)⁠. Having computed Blist, bcajack2 is invoked as bcajack2(Blist) without need to enter the function func.

Usage

bcajack2(
  x,
  B,
  func,
  ...,
  m = nrow(x),
  mr,
  pct = 0.333,
  K = 2,
  J = 12,
  alpha = c(0.025, 0.05, 0.1, 0.16),
  verbose = TRUE
)

Arguments

x

an n \times p data matrix, rows are observed p-vectors, assumed to be independently sampled from target population. If p is 1 then x can be a vector.

B

number of bootstrap replications. B can also be a vector of B bootstrap replications of the estimated parameter of interest, computed separately. If B is Blist as explained above, x is not needed.

func

function \hat{\theta}=func(x) computing estimate of the parameter of interest; func(x) should return a real value for any n^\prime \times p matrix x^\prime, n^\prime not necessarily equal to n

...

additional arguments for func.

m

an integer less than or equal to n; the routine collects the n rows of x into m groups to speed up the jackknife calculations for estimating the acceleration value a; typically m is 20 or 40 and does not have to exactly divide n. However, warnings will be shown.

mr

if m < n then mr repetions of the randomly grouped jackknife calculations are averaged.

pct

bcajack2 uses those count vectors nearest (1,1,...1) to estimate the gradient of the statistic, "nearest" being defined as those count vectors in the smallest pct of all B of them. Default value for 'pct is 1/3 (see appendix in Efron and Narasimhan for further details)

K

a non-negative integer. If K > 0, bcajack also returns estimates of internal standard error, that is, of the variability due to stopping at B bootstrap replications rather than going on to infinity. These are obtained from a second type of jackknifing, taking an average of K separate jackknife estimates, each randomly splitting the B bootstrap replications into J groups.

J

the number of groups into which the bootstrap replications are split

alpha

percentiles desired for the bca confidence limits. One only needs to provide alpha values below 0.5; the upper limits are automatically computed

verbose

logical for verbose progress messages

Value

a named list of several items

• lims : first column shows the estimated bca confidence limits at the requested alpha percentiles. These can be compared with the standard limits \hat{\theta} + \hat{\sigma}z_{\alpha}, third column. The second column jacksd gives the internal standard errors for the bca limits, quite small in the example. Column 4, pct, gives the percentiles of the ordered B bootstrap replications corresponding to the bca limits, eg the 897th largest replication equalling the .975 bca limit .557.

• stats : top line of stats shows 5 estimates: theta is func(x), original point estimate of the parameter of interest; sdboot is its bootstrap estimate of standard error; z0 is the bca bias correction value, in this case quite negative; a is the acceleration, a component of the bca limits (nearly zero here); sdjack is the jackknife estimate of standard error for theta. Bottom line gives the internal standard errors for the five quantities above. This is substantial for z0 above.

• B.mean : bootstrap sample size B, and the mean of the B bootstrap replications \hat{\theta^*}

• ustats : The bias-corrected estimator 2 * t0 - mean(tt), and an estimate sdu of its sampling error

• seed : The random number state for reproducibility

Examples

data(diabetes, package = "bcaboot")
Xy <- cbind(diabetes$x, diabetes$y)
rfun <- function(Xy) {
  y <- Xy[, 11]
  X <- Xy[, 1:10]
  summary(lm(y~X) )$adj.r.squared
}
set.seed(1234)
bcajack2(x = Xy, B = 1000, func = rfun, m = 40, verbose = FALSE)