confirm: Confirm that Clustering in Covariate X-space yields an...

In LocalControlStrategy: Local Control Strategy for Robust Analysis of Cross-Sectional Data

Confirm that Clustering in Covariate X-space yields an "adjusted" LTD/LRC effect-size Distribution

Description

For a given Number of Clusters, K, confirm() compares the observed distribution of LTDs or LRCs from relatively well-matched experimental units with the corresponding distribution from Purely Random Clusterings of experimental units. The larger are differences between the (blue) observed empirical CDF of effect-sizes and the (red) Purely Random CDF, the more potentially IMPORTANT are the "adjustments" resulting from focussing upon clustering (matching) of experimental units in X-space.

Usage

confirm(x, reps=100, seed=12345)

Arguments

x	An output object from ltdagg() or lrcagg() for a specified number of clusters, K.
reps	Number of simulation Replications, each with the same number, K, and sizes, N1, N2, ..., NK of Purely Random clusters.
seed	This (arbitrary) integer argument will be passed to the R set.seed() function. Knowing the value of this seed makes the output from confirm() reproducible.

Details

Making calls to confirm() for ltdagg() or lrcagg() objects resulting from different choices of K = Numbers of Clusters help the analyst decide which observed LTD or LRC effect-size distributions are (or are not) meaningfully different from Purely Random. When the X-covariates used in LCcluster() are truly "ignorable," then [i] all X-based clusters will be Purely Random, and [ii] both the number (K) and the sizes (N1, N2, ...,NK) of clusters formed will be meaningless and arbitrary. Thus the LC Strategy confirm() function simulates the empirical CDF for LTDs or LRCs resulting from purely random permutations of the Cluster ID numbers (1, 2, ...,K) assigned by ltdagg() or lrcagg(). Each permutation yields K artificial "clusters" of sizes N1, N2, ..., NK. Simulation results are accumulated for the total number of random permutations specified in the "reps=" argument of confirm(). Calls to print.confirm() and plot.confirm() provide information on comparisons of empirical CDFs for the Observed and Purely Random LTD/LRC distributions, including calculation of an observed two-sample Kolmogorov-Smirnov D-statistic using stats::ks.test. This is a non-standard use of ks.test() because the distributions being compared are DISCRETE; both contain many within-cluster TIED effect-size estimates. The p-value computed by ks.test() is not reported or saved because it is badly biased downwards due to TIED estimates. Researchers wishing to simulate a p-value for the observed KS D-statistic that is adjusted for TIES can invoke KSperm(confirm()).

Value

An output list object of class confirm:

hiclus	Hierarchical clustering object created by the designated method.
dframe	Name of data.frame containing X, trex & Y variables.
trtm	Name of numerical trex variable.
yvar	Name of numerical Y-outcome variable.
reps	Number of overall Replications, each with the same numbers of requested clusters.
seed	Integer argument passed to set.seed(). Knowing which seed value was used in the call to confirm() makes not only the NULL distribution of observed LTDs or LRCs reproducible but also makes the NULL distribution of D-statistics (adjusted for ties) from a subsequent call to KSperm() reproducible.
nclus	Number of clusters requested.
units	Number of experimental units or patients.
Type	1 ==> LTDs, otherwise LRCs.
LCmean	Weighted Local Mean across Clusters.
LCstde	Weighted Std. Error across Clusters.
RPmean	Weighted Random Permutation Mean across Clusters.
RPstde	Weighted Random Permutation Std. Error across Clusters.
KSobsD	Output from print(ks.test()).
LCdist	data.frame of 5 key variables for all experimental units.
dfconf	data.frame of lstat = LTD or LRC values of max(length) = reps*units.

Author(s)

Bob Obenchain <[e-mail]>

References

Obenchain RL. (2010) The Local Control Approach using JMP. Chapter 7 of Analysis of Observational Health Care Data using SAS, Cary, NC:SAS Press, pages 151-192.

Obenchain RL. (2019) LCstrategy_in_R.pdf http://localcontrolstatistics.org

See Also

ltdagg and lrcagg.

LocalControlStrategy documentation built on Nov. 10, 2022, 5:49 p.m.