tweights {tboot} | R Documentation |
tweights
Returns a vector p
of resampling probabilities
such that the column means of tboot(dataset = dataset, p = p)
equals target
on average.
tweights(dataset, target = apply(dataset, 2, mean), distance = "klqp",
maxit = 1000, tol = 1e-08, warningcut = 0.05, silent = FALSE,
Nindependent = 0)
dataset |
Data frame or matrix to use to find row weights. |
target |
Numeric vector of target column means. If the 'target' is named, then all elements of names(target) should be in the dataset. |
distance |
The distance to minimize. Must be either 'euchlidean,' 'klqp' or 'klpq' (i.e. Kullback-Leibler). 'klqp' which is exponential tilting is recommended. |
maxit |
Defines the maximum number of iterations for optimizing 'kl' distance. |
tol |
Tolerance. If the achieved mean is to0 far from the target (i.e. as defined by tol) an error will be thrown. |
warningcut |
Sets the cutoff for determining when a large weight will trigger a warning. |
silent |
Allows silencing of some messages. |
Nindependent |
Assumes the input also includes 'Nindependent' samples with independent columns. See details. |
Let p_i = 1/n
be the probability of sampling subject i
from a dataset with n
individuals (i.e. rows of the dataset) in the classic resampling with replacement scheme.
Also, let q_i
be the probability of sampling subject i
from a dataset with n
individuals in our new resampling scheme. Let d(q,p)
represent a distance between the two resampling schemes. The tweights
function seeks to solve the problem:
q = argmin_p d(q,p)
Subject to the constraint that:
sum_i q_i = 1
and
dataset' q = target
where dataset is a n x K matrix of variables input to the function.
d_{euclidian}(q,p) = sqrt( \sum_i (p_i-q_i)^2 )
d_{kl}(q,p) = \sum_i (log(p_i) - log(q_i))
Optimization for Euclidean distance is a quadratic program and utilizes the ipop function in kernLab. Optimization for the others utilize a Newton-Raphson type iterative algorithm.
If the original target cannot be achieved. Something close to the original target will be selected. A warning will be produced and the new target displayed.
The 'Nindependent' option augments the dataset by assuming some additional specified number of patients. These patients are assumed to made up of a random bootstrapped sample from the dataset for each variable marginally leading to independent variables.
An object of type tweights
. This object contains the following components:
Tilted weights for resampling
Will be null if target was not changed.
Actual target that was attempted.
Achieved mean from tilting.
Inputed dataset.
Reformated dataset.
Inputed 'Nindependent' option.
target=c(Sepal.Length=5.5, Sepal.Width=2.9, Petal.Length=3.4)
w = tweights(dataset = iris, target = target, silent = TRUE)
simulated_data = tboot(nrow = 1000, weights = w)