ldPower: Power and Other Probabilities for Group Sequential Boundaries

In ldbounds: Lan-DeMets Method for Group Sequential Boundaries

Power and Other Probabilities for Group Sequential Boundaries

Description

'ldPower' calculates drift (effect), confidence interval for drift, adjusted p-value, or power and other probabilities given drift for specified group sequential boundaries for interim analyses of accumulating data in clinical trials.

Usage

ldPower(t, za=NULL, zb=NULL, t2=t, pow=NULL, drift=NULL,
      conf=NULL, method=NULL, pvaltime=NULL,
      zval=zb[length(zb)])

Arguments

t	a vector of analysis times or an 'ldBounds' object (from either the 'ldBounds' or 'commonbounds' function). If a vector of analysis times, must be increasing and in (0,1].
za	the vector of lower boundaries. If not specified, made symmetric to zb.
zb	the vector of upper boundaries.
t2	the second time scale, usually in terms of amount of accumulating information. By default, same as t.
pow	the desired power when drift is not specified.
drift	the true drift (i.e. treatment effect when t=1). Default is 0 when pow, conf, and method are also left unspecified.
conf	the confidence level when a confidence interval for drift is wanted.
method	the type of adjusted p-value desired. Possible values are 'SW' (stage-wise) and 'LR' (likelihood ratio).
pvaltime	the analysis time at which the final Z-statistic was observed and an adjusted p-value is desired.
zval	the final observed Z-statistic (i.e. when trial is stopped). Used for confidence interval or ajusted p-value. Default is final upper boundary value.

Details

This is based on a Fortran program, 'ld98', by Reboussin, DeMets, Kim, and Lan. It has some advantages, like making use of probability distributions in R. Only one of pow, drift, conf, or pval is to be specified and zval is only used in the latter two cases.

If t is an 'ldBounds' object, za, zb, t, and t2 are already defined and should not be specified.

Value

'ldPower' returns an object of 'class' '"ldPower"'.

An object of class '"ldPower"' is a list containing the following components:

type	Type of computation performed: 1 is drift given power, 2 is exit probabilities given drift, 3 is confidence interval for drift given final Z-statistic, and 4 is adjusted p-value given final Z-statistic.
time	the original time scale.
time2	the second (information) time scale.
lower.bounds	the vector of lower boundaries given.
upper.bounds	the vector of upper boundaries given.
power	the power. If power is given, it is returned here. If drift is given, the resulting power is calculated.
drift	the drift. If drift is given, it is returned here. If power is given, the drift resulting in given power is calculated.
lower.probs	the vector of exit probabilities across the lower boundary. Returned if power or drift is given.
upper.probs	the same for upper boundary.
exit.probs	the probability at each analysis of crossing the boundary. The sum of lower.probs and upper.probs.
cum.exit	the cumulative probability of crossing.
conf.level	the desired confidence level, if given.
final.zvalue	the final Z statistic, if given.
conf.interval	the confidence interval for drift, if conf and zval are given.
p.ordering	the ordering specified for p-value calculation (if given).
p.value	the adjusted p-value if pval, pvaltime, and zval are given.

Author(s)

Charlie Casper [e-mail], Thomas Cook [e-mail], and Oscar A. Perez

References

Reboussin, D. M., DeMets, D. L., Kim, K. M., and Lan, K. K. G. (2000) Computations for group sequential boundaries using the Lan-DeMets spending function method. Controlled Clinical Trials, 21:190-207.

Fortran program 'ld98' by the same authors as above.

DeMets, D. L. and Lan, K. K. G. (1995) Recent Advances in Clinical Trial Design and Analysis, Thall, P. F. (ed.). Boston: Kluwer Academic Publishers.

Lan, K. K. G. and DeMets, D. L. (1983) Discrete sequential boundaries for clinical trials. Biometrika, 70:659-63.

See Also

Generic functions summary.ldPower and plot.ldPower.

ldBounds for computation of boundaries using alpha spending function method.

commonbounds for boundaries that do not use alpha spending.

Examples

   ## From Reboussin, et al. (2000)

   t <- c(0.13,0.4,0.69,0.9,0.98,1)
   upper <- c(5.3666,3.7102,2.9728,2.5365,2.2154,1.9668)
   bound.pr <- ldPower(t,zb=upper,drift=3.242)
   summary(bound.pr)

   t <- c(0.2292,0.3333,0.4375,0.5833,0.7083,0.8333)
   power.fam <- ldBounds(t,iuse=3,alpha=0.05)
   bound.ci <- ldPower(power.fam,conf=0.95,zval=2.82)
   bound.p <- ldPower(power.fam,method="LR",pvaltime=5,zval=2.82)
   summary(bound.ci)
   summary(bound.p)
   plot(bound.ci)

   obf.bd <- ldBounds(5)
   obf.dr <- ldPower(obf.bd,pow=0.9)
   summary(obf.dr)

ldbounds documentation built on June 22, 2024, 6:53 p.m.