netest {EpiModel}

Dynamic Network Model Estimation

Description

Estimates statistical network models using the exponential random graph modeling (ERGM) framework with extensions for dynamic/temporal models (STERGM).

Usage

netest(
  nw,
  formation,
  target.stats,
  coef.diss,
  constraints,
  coef.form = NULL,
  edapprox = TRUE,
  set.control.ergm = control.ergm(),
  set.control.tergm = control.tergm(),
  set.control.ergm.ego = control.ergm.ego(),
  verbose = FALSE,
  nested.edapprox = TRUE,
  ...
)

Arguments

nw

An object of class network or egor, with the latter indicating an ergm.ego fit.

formation

Right-hand sided STERGM formation formula in the form ~edges + ..., where ... are additional network statistics.

target.stats

Vector of target statistics for the formation model, with one number for each network statistic in the model. Ignored if fitting via ergm.ego.

coef.diss

An object of class disscoef output from the dissolution_coefs function.

constraints

Right-hand sided formula specifying constraints for the modeled network, in the form ~..., where ... are constraint terms. By default, no constraints are set.

coef.form

Vector of coefficients for the offset terms in the formation formula.

edapprox

If TRUE, use the indirect edges dissolution approximation method for the dynamic model fit, otherwise use the more time-intensive full STERGM estimation (see details). For nw of class egor, only edapprox = TRUE is supported.

set.control.ergm

Control arguments passed to ergm (see details).

set.control.tergm

Control arguments passed to tergm (see details).

set.control.ergm.ego

Control arguments passed to ergm.ego (see details).

verbose

If TRUE, print model fitting progress to console.

nested.edapprox

Logical. If edapprox = TRUE the dissolution model is an initial segment of the formation model (see details).

...

Additional arguments passed to other functions.

Details

netest is a wrapper function for the ergm, ergm.ego, and tergm functions that estimate static and dynamic network models. Network model estimation is the first step in simulating a stochastic network epidemic model in EpiModel. The output from netest is a necessary input for running the epidemic simulations in netsim. With a fitted network model, one should always first proceed to model diagnostics, available through the netdx function, to check model fit. A detailed description of fitting these models, along with examples, may be found in the Basic Network Models tutorials.

Value

A fitted network model object of class netest.

Edges Dissolution Approximation

The edges dissolution approximation method is described in Carnegie et al. This approximation requires that the dissolution coefficients are known, that the formation model is being fit to cross-sectional data conditional on those dissolution coefficients, and that the terms in the dissolution model are a subset of those in the formation model. Under certain additional conditions, the formation coefficients of a STERGM model are approximately equal to the coefficients of that same model fit to the observed cross-sectional data as an ERGM, minus the corresponding coefficients in the dissolution model. The approximation thus estimates this ERGM (which is typically much faster than estimating a STERGM) and subtracts the dissolution coefficients.

The conditions under which this approximation best hold are when there are few relational changes from one time step to another; i.e. when either average relational durations are long, or density is low, or both. Conveniently, these are the same conditions under which STERGM estimation is slowest. Note that the same approximation is also used to obtain starting values for the STERGM estimate when the latter is being conducted. The estimation does not allow for calculation of standard errors, p-values, or likelihood for the formation model; thus, this approach is of most use when the main goal of estimation is to drive dynamic network simulations rather than to conduct inference on the formation model. The user is strongly encouraged to examine the behavior of the resulting simulations to confirm that the approximation is adequate for their purposes. For an example, see the vignette for the package tergm.

It has recently been found that subtracting a modified version of the dissolution coefficients from the formation coefficients provides a more principled approximation, and this is now the form of the approximation applied by netest. The modified values subtracted from the formation coefficients are equivalent to the (crude) dissolution coefficients with their target durations increased by 1. The nested.edapprox argument toggles whether to implement this modified version by appending the dissolution terms to the formation model and appending the relevant values to the vector of formation model coefficients (value = FALSE), whereas the standard version subtracts the relevant values from the initial formation model coefficients (value = TRUE).

Control Arguments

The ergm, ergm.ego, and tergm functions allow control settings for the model fitting process. When fitting a STERGM directly (setting edapprox to FALSE), control parameters may be passed to the tergm function with the set.control.tergm argument in netest. The controls should be input through the control.tergm() function, with the available parameters listed in the control.tergm help page in the tergm package.

When fitting a STERGM indirectly (setting edapprox to TRUE), control settings may be passed to the ergm function using set.control.ergm, or to the ergm.ego function using set.control.ergm.ego. The controls should be input through the control.ergm() and control.ergm.ego() functions, respectively, with the available parameters listed in the control.ergm help page in the ergm package and the control.ergm.ego help page in the ergm.ego package. An example is below.

References

Krivitsky PN, Handcock MS. "A separable model for dynamic networks." JRSS(B). 2014; 76.1:29-46.

Carnegie NB, Krivitsky PN, Hunter DR, Goodreau SM. An approximation method for improving dynamic network model fitting. Journal of Computational and Graphical Statistics. 2014; 24(2): 502-519.

Jenness SM, Goodreau SM and Morris M. EpiModel: An R Package for Mathematical Modeling of Infectious Disease over Networks. Journal of Statistical Software. 2018; 84(8): 1-47.

See Also

Use netdx to diagnose the fitted network model, and netsim to simulate epidemic spread over a simulated dynamic network consistent with the model fit.

Examples

# Initialize a network of 100 nodes
nw <- network_initialize(n = 100)

# Set formation formula
formation <- ~edges + concurrent

# Set target statistics for formation
target.stats <- c(50, 25)

# Obtain the offset coefficients
coef.diss <- dissolution_coefs(dissolution = ~offset(edges), duration = 10)

# Estimate the STERGM using the edges dissolution approximation
est <- netest(nw, formation, target.stats, coef.diss,
              set.control.ergm = control.ergm(MCMC.burnin = 1e5,
                                              MCMC.interval = 1000))
est

# To estimate the STERGM directly, use edapprox = FALSE
# est2 <- netest(nw, formation, target.stats, coef.diss, edapprox = FALSE)