| bMCTtest {DoseFinding} | R Documentation |
Performs Bayesian multiple contrast test
Description
This function performs a Bayesian multiple contrast test using normal mixture
priors for the response on each dose, as proposed in Fleischer et al. (2022). For a general description of the
multiple contrast test see MCTtest.
If ‘type = "normal"’, an ANCOVA model based on a homoscedastic normality assumption is fitted and posteriors for dose-response and contrast vectors are obtained assuming a known variance.
For ‘type = "general"’ it is assumed multivariate normally distributed estimates are specified in ‘resp’ with covariance given by ‘S’, which define the likelihood. Posteriors for dose-response and contrast vectors are then obtained assuming a known covariance matrix S
The multiple contrast test decision is based on the maximum posterior probability of a contrast being greater than zero. Thresholds for the posterior probability can either be supplied or will be derived from frequentist critical values. In the latter case the Bayesian test will give approximately the same results as the frequentist multiple contrast test if uninformative priors are used.
Usage
bMCTtest(dose, resp, data = NULL, models, S = NULL, type = c("normal", "general"),
prior, alpha = 0.025, na.action = na.fail, mvtcontrol = mvtnorm.control(),
contMat = NULL, critV = NULL)
Arguments
dose, resp |
Either vectors of equal length specifying dose and response values, or names of variables in the data frame specified in ‘data’. |
data |
Data frame containing the variables referenced in dose and resp if ‘data’ is not specified it is assumed that ‘dose’ and ‘resp’ are variables referenced from data (and no vectors) |
models |
An object of class ‘Mods’, see |
S |
The covariance matrix of ‘resp’ when ‘type = "general"’, see Description. |
type |
Determines whether the likelihood is based on an ANCOVA model under a
homoscedastic normality assumption (when ‘type = "normal"’), or
estimates at the doses and their covariance matrix are specified
directly in ‘resp’ and ‘S’. See also |
prior |
List of length equal to the number of doses with the prior for each arm. Each element needs to be of class ‘normMix’ (See ‘RBesT’ package documentation). It is assumed that the i-th component of the prior list corresponds to the i-th largest dose. For example the first entry in the list is the prior for the placebo group, the second entry the prior for the second lowest dose and so on. Internally the priors across the different arms are combined (densities multiplied) assuming independence. The resulting multivariate normal mixture prior will have as many components as the product of the number of components of the individual mixture priors. The posterior mixture is part of the result object under "posterior". |
alpha |
Significance level for the frequentist multiple contrast test. If no critical values are supplied via ‘critV’ this is used to derive critical values for Bayesian decision rule. |
na.action |
A function which indicates what should happen when the data contain NAs. |
mvtcontrol |
A list specifying additional control parameters for the ‘qmvt’
and ‘pmvt’ calls in the code used to obtain frequentist critical values,
see also |
contMat |
Contrast matrix to apply to the posterior dose-response estimates. The
contrasts need to be in the columns of the matrix (i.e. the column
sums need to be 0). If not specified optimal contrasts are
calculated using |
critV |
Supply a critical value for the maximum posterior probability of the contrasts being greater than zero that needs to be surpassed to establish a non-flat dose-response. If this argument is NULL, this will be derived from critical values for frequentist MCP-Mod using the provided ‘alpha’. |
Details
For the default calculation of optimal contrasts the prior information is ignored (i.e. contrasts are calculated
in the same way as in MCTtest).
Fleischer et al. (2022) discuss using contrasts that take the prior effective sample sizes into account, which can be
slightly more favourable for the Bayesian MCT test. Such alternative contrasts can be directly handed over via
the ‘contMat’ argument.
For analysis with covariate adjustment, covariate-adjusted ‘resp’ and ‘S’ can be supplied together with using ‘type = "general"’. See the vignette "Design and analysis template MCP-Mod for binary data" for an example on how to obtain covariate adjusted estimates.
Value
An object of class bMCTtest, a list containing the output.
Author(s)
Marius Thomas
References
Fleischer, F., Bossert, S., Deng, Q., Loley, C. and Gierse, J. (2022). Bayesian MCP-Mod, Pharmaceutical Statistics, 21, 654–670
See Also
Examples
require(RBesT)
###############################
## Normal outcome
###############################
data(biom)
## define shapes for which to calculate optimal contrasts
doses <- c(0, 0.05, 0.2, 0.6, 1)
modlist <- Mods(emax = 0.05, linear = NULL, logistic = c(0.5, 0.1),
linInt = c(0, 1, 1, 1), doses = doses)
## specify an informative prior for placebo, weakly informative for other arms
plc_prior <- mixnorm(inf = c(0.8, 0.4, 0.1), rob = c(0.2, 0.4, 10))
vague_prior <- mixnorm(c(1, 0, 10))
## i-th component of the prior list corresponds to the i-th largest dose
## (e.g. 1st component -> placebo prior; last component prior for top dose)
prior <- list(plc_prior, vague_prior, vague_prior, vague_prior, vague_prior)
m1 <- bMCTtest(dose, resp, biom, models=modlist, prior = prior)
## now supply a critical value (= threshold for maxmimum posterior probability)
m2 <- bMCTtest(dose, resp, biom, models=modlist, prior = prior, critV = 0.99)
####################################
## Binary outcome with covariates
####################################
## generate data
logit <- function(p) log(p / (1 - p))
inv_logit <- function(y) 1 / (1 + exp(-y))
doses <- c(0, 0.5, 1.5, 2.5, 4)
## set seed and ensure reproducibility across R versions
set.seed(1, kind = "Mersenne-Twister", sample.kind = "Rejection", normal.kind = "Inversion")
group_size <- 100
dose_vector <- rep(doses, each = group_size)
N <- length(dose_vector)
## generate covariates
x1 <- rnorm(N, 0, 1)
x2 <- factor(sample(c("A", "B"), N, replace = TRUE, prob = c(0.6, 0.4)))
## assume approximately logit(10%) placebo and logit(35%) asymptotic response with ED50=0.5
prob <- inv_logit(emax(dose_vector, -2.2, 1.6, 0.5) + 0.3 * x1 + 0.3 * (x2 == "B"))
dat <- data.frame(y = rbinom(N, 1, prob),
dose = dose_vector, x1 = x1, x2 = x2)
## specify an informative prior for placebo (on logit scale), weakly informative for other arms
plc_prior <- mixnorm(inf = c(0.8, -2, 0.5), rob = c(0.2, -2, 10))
vague_prior <- mixnorm(c(1, 0, 10))
prior <- list(plc_prior, vague_prior, vague_prior, vague_prior, vague_prior)
## candidate models
mods <- Mods(emax = c(0.25, 1), sigEmax = rbind(c(1, 3), c(2.5, 4)), betaMod = c(1.1, 1.1),
placEff = logit(0.1), maxEff = logit(0.35)-logit(0.1),
doses = doses)
fit_cov <- glm(y~factor(dose) + 0 + x1 + x2, data = dat, family = binomial)
covariate_adjusted_estimates <- function(mu_hat, S_hat, formula_rhs,
doses, other_covariates, n_sim) {
## predict every patient under *every* dose
oc_rep <- as.data.frame(lapply(other_covariates, function(col) rep(col, times = length(doses))))
d_rep <- rep(doses, each = nrow(other_covariates))
pdat <- cbind(oc_rep, dose = d_rep)
X <- model.matrix(formula_rhs, pdat)
## average on probability scale then backtransform to logit scale
mu_star <- logit(tapply(inv_logit(X %*% mu_hat), pdat$dose, mean))
## estimate covariance matrix of mu_star
pred <- replicate(n_sim, logit(tapply(inv_logit(X %*% drop(rmvnorm(1, mu_hat, S_hat))),
pdat$dose, mean)))
return(list(mu_star = as.numeric(mu_star), S_star = cov(t(pred))))
}
ca <- covariate_adjusted_estimates(coef(fit_cov), vcov(fit_cov), ~factor(dose)+0+x1+x2,
doses, dat[, c("x1", "x2")], 1000)
bMCTtest(doses, ca$mu_star, S = ca$S_star, type = "general", models = mods, prior = prior)
################################################
## example with contrasts handed over
################################################
data(biom)
## define shapes for which to calculate optimal contrasts
doses <- c(0, 0.05, 0.2, 0.6, 1)
modlist <- Mods(emax = 0.05, linear = NULL, sigEmax = c(0.5, 5),
linInt = c(0, 1, 1, 1), doses = doses)
## specify an informative prior for placebo, weakly informative for other arms
plc_prior <- mixnorm(inf = c(0.8, 0.4, 0.1), rob = c(0.2, 0.4, 10), sigma = 0.7)
vague_prior <- mixnorm(c(1, 0, 10), sigma = 0.7)
prior <- list(plc_prior, vague_prior, vague_prior, vague_prior, vague_prior)
## use prior effective sample sizes to calculate optimal contrasts
prior_ess <- unlist(lapply(prior, ess))
n_grp <- as.numeric(table(biom$dose))
weights <- n_grp + prior_ess
cmat <- optContr(modlist, w = weights)
bMCTtest(dose, resp, biom, models=modlist, prior = prior, contMat = cmat)