tilt.boot                package:boot                R Documentation

Non-parametric Tilted Bootstrap

Description:

     This function will run an initial bootstrap with equal resampling 
     probabilities (if required) and will use the output of the initial
     run to  find resampling probabilities which put the value of the
     statistic at required values.  It then runs an importance
     resampling bootstrap using the calculated probabilities as the
     resampling distribution.

Usage:

     tilt.boot(data, statistic, R, sim="ordinary", stype="i", 
               strata=rep(1, n), L=NULL, theta=NULL, 
               alpha=c(0.025, 0.975), tilt=TRUE, width=0.5, 
               index=1, ...)

Arguments:

    data: The data as a vector, matrix or data frame.  If it is a
          matrix or data frame  then each row is considered as one
          (multivariate) observation. 

statistic: A function which when applied to data returns a vector
          containing the statistic(s) of interest.  It must take at
          least two arguments.  The first argument will always be
          'data' and the second should be a vector of indices, weights
          or frequencies describing the bootstrap sample.  Any other
          arguments must be supplied to 'tilt.boot' and will be passed
          unchanged to statistic each time it is called. 

       R: The number of bootstrap replicates required.  This will
          generally be a vector, the first value stating how many
          uniform bootstrap simulations are to be  performed at the
          initial stage.  The remaining values of 'R' are the number of
           simulations to be performed resampling from each reweighted
          distribution.   The first value of 'R' must always be
          present, a value of 0 implying that no uniform resampling is
          to be carried out.  Thus 'length(R)' should always equal
          '1+length(theta)'. 

     sim: This is a character string indicating the type of bootstrap
          simulation required. There are only two possible values that
          this can take: '"ordinary"' and '"balanced"'.  If other
          simulation types are required for the initial un-weighted
          bootstrap then it will be necessary to run 'boot', calculate
          the weights appropriately, and run 'boot' again using the
          calculated weights. 

   stype: A character string indicating the type of second argument
          expected by  'statistic'.  The possible values that 'stype'
          can take are '"i"' (indices),  '"w"' (weights) and '"f"'
          (frequencies). 

  strata: An integer vector or factor representing the strata for
          multi-sample problems. 

       L: The empirical influence values for the statistic of interest.
           They are used only for exponential tilting when 'tilt' is
          'TRUE'.  If 'tilt' is 'TRUE' and  they are not supplied then
          'tilt.boot' uses 'empinf' to calculate them. 

   theta: The required parameter value(s) for the tilted
          distribution(s).  There should be one value of 'theta' for
          each of the non-uniform distributions.  If 'R[1]' is 0
          'theta' is a required argument.  Otherwise 'theta' values can
          be estimated  from the initial uniform bootstrap and the
          values in 'alpha'. 

   alpha: The alpha level to which tilting is required.  This parameter
          is ignored if 'R[1]' is 0 or if 'theta' is supplied,
          otherwise it is used to find the values  of 'theta' as
          quantiles of the initial uniform bootstrap.  In this case
          'R[1]'  should  be large enough that 'min(c(alpha,
          1-alpha))*R[1] > 5', if this is not  the case then a warning
          is generated to the effect that the 'theta' are extreme 
          values and so the tilted output may be unreliable.   

    tilt: A logical variable which if 'TRUE' (the default) indicates
          that exponential  tilting should be used, otherwise local
          frequency smoothing ('smooth.f') is  used.  If 'tilt' is
          'FALSE' then 'R[1]' must be positive.  In fact in this case 
          the value of 'R[1]' should be fairly large (in the region of
          500 or more). 

   width: This argument is used only if 'tilt' is 'FALSE', in which
          case it is passed  unchanged to 'smooth.f' as the
          standardized bandwidth for the smoothing  operation.  The
          value should generally be in the range (0.2, 1). See
          'smooth.f' for for more details. 

   index: The index of the statistic of interest in the output from
          'statistic'.  By  default the first element of the output of
          'statistic' is used. 

     ...: Any additional arguments required by 'statistic'.  These are
          passed unchanged to 'statistic' each time it is called. 

Value:

     An object of class '"boot"' with the following components

      t0: The observed value of the statistic on the original data. 

       t: The values of the bootstrap replicates of the statistic. 
          There will be 'sum(R)' of these, the first 'R[1]'
          corresponding to the uniform bootstrap and the  remainder to
          the tilted bootstrap(s). 

       R: The input vector of the number of bootstrap replicates. 

    data: The original data as supplied. 

statistic: The 'statistic' function as supplied. 

     sim: The simulation type used in the bootstrap(s), it can either
          be '"ordinary"' or '"balanced"'. 

   stype: The type of statistic supplied, it is the same as the input
          value 'stype'. 

    call: A copy of the original call to 'tilt.boot'. 

  strata: The strata as supplied. 

 weights: The matrix of weights used.  If 'R[1]' is greater than 0 then
          the first row  will be the uniform weights and each
          subsequent row the tilted weights.   If 'R[1]' equals 0 then
          the uniform weights are omitted and only the tilted weights
          are output. 

   theta: The values of 'theta' used for the tilted distributions. 
          These are either the input values or the values derived from
          the uniform bootstrap and 'alpha'. 

References:

     Booth, J.G., Hall, P. and Wood, A.T.A. (1993) Balanced importance
     resampling  for the bootstrap. _Annals of Statistics_, *21*,
     286-298.

     Davison, A.C. and Hinkley, D.V. (1997)  _Bootstrap Methods and
     Their Application_. Cambridge University Press.

     Hinkley, D.V. and Shi, S. (1989) Importance sampling and the
     nested bootstrap.  _Biometrika_, *76*, 435-446.

See Also:

     'boot', 'exp.tilt', 'Imp.Estimates', 'imp.weights', 'smooth.f'

Examples:

     # Note that these examples can take a while to run.

     # Example 9.9 of Davison and Hinkley (1997).
     grav1 <- gravity[as.numeric(gravity[,2])>=7,]
     grav.fun <- function(dat, w, orig)
     {    strata <- tapply(dat[, 2], as.numeric(dat[, 2]))
          d <- dat[, 1]
          ns <- tabulate(strata)
          w <- w/tapply(w, strata, sum)[strata]
          mns <- tapply(d * w, strata, sum)
          mn2 <- tapply(d * d * w, strata, sum)
          s2hat <- sum((mn2 - mns^2)/ns)
          c(mns[2]-mns[1],s2hat,(mns[2]-mns[1]-orig)/sqrt(s2hat))
     }
     grav.z0 <- grav.fun(grav1,rep(1,26),0)
     tilt.boot(grav1, grav.fun, R=c(249,375,375), stype="w", 
               strata=grav1[,2], tilt=TRUE, index=3, orig=grav.z0[1]) 

     #  Example 9.10 of Davison and Hinkley (1997) requires a balanced 
     #  importance resampling bootstrap to be run.  In this example we 
     #  show how this might be run.  
     acme.fun <- function(data, i, bhat)
     {    d <- data[i,]
          n <- nrow(d)
          d.lm <- glm(d$acme~d$market)
          beta.b <- coef(d.lm)[2]
          d.diag <- glm.diag(d.lm)
          SSx <- (n-1)*var(d$market)
          tmp <- (d$market-mean(d$market))*d.diag$res*d.diag$sd
          sr <- sqrt(sum(tmp^2))/SSx
          c(beta.b, sr, (beta.b-bhat)/sr)
     }
     acme.b <- acme.fun(acme,1:nrow(acme),0)
     acme.boot1 <- tilt.boot(acme, acme.fun, R=c(499, 250, 250), 
                             stype="i", sim="balanced", alpha=c(0.05, 0.95), 
                             tilt=TRUE, index=3, bhat=acme.b[1])

