In statnet/tergm: Fit, Simulate and Diagnose Models for Network Evolution Based on Exponential-Family Random Graph Models

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Introduction

Version 4.0 of the tergm package introduces new user interfaces for specifying tergm models. While an effort has been made to maintain a high degree of backwards compatibility, there are some points of backwards incompatibility, and some users may wish to convert their code to use the new interfaces anyway, so this document describes how to go about doing that. The examples given here are somewhat artificial so as to better illustrate the range of possible changes needed; they may not be typical or even plausible in every detail, but are intended to exhibit the types of updates that users may need to make.

Estimation

Estimation calls in tergm 3.x might look something like

data(samplk)
samp <- list(samplk1, samplk2, samplk3)
samp.fit <- stergm(samp,
                   formation = ~edges+mutual+cyclicalties+transitiveties,
                   dissolution = ~edges+mutual+cyclicalties+transitiveties,
                   estimate = "CMLE",
                   times = 1:3,
                   control = control.stergm(CMLE.control.form = control.ergm(init = c(-3.5,2,0,NA)),
                                            CMLE.control.diss = control.ergm(init = c(0,1,0,1/2))))

for CMLE, and

data(florentine)
stergm.fit.1 <- stergm(flobusiness,
                       formation = ~edges+gwesp(0,fixed=T),
                       dissolution = ~offset(edges),
                       targets = "formation",
                       offset.coef.diss = log(9),
                       estimate = "EGMME",
                       control = control.stergm(SA.plot.progress=TRUE))

for EGMME.

To convert these to the new 4.0 user interface, we make the following changes.

• Replace the function name stergm with tergm (in 4.0, the tergms need not be separable, hence we drop the s).

• Combine the network (or network list), formation, and dissolution formulas into a single formula, schematically of the form

network ~ Form(formation formula) + Persist(dissolution formula)

where Form and Persist are operator terms defined in tergm 4.0.

For the CMLE example, this results in the formula

samp ~ Form(~edges+mutual+cyclicalties+transitiveties) + Persist(~edges+mutual+cyclicalties+transitiveties)

and for the EGMME example, it results in the formula

flobusiness ~ Form(~edges+gwesp(0,fixed=T)) + Persist(~offset(edges))

These formulas will be our first arguments to the tergm function.

• The control argument (if present), previously of class control.stergm, should be replaced by one of class control.tergm. This can be accomplished by replacing control.stergm() with control.tergm(), snctrl(), or list(), and updating arguments as follows. Arguments to control.stergm() occurring in pairs with .form and .diss in their names have been collapsed to single, correspondingly named arguments to control.tergm() without .form or .diss. Additionally, the arguments CMLE.control.form and CMLE.control.diss to control.stergm() correspond to the CMLE.ergm argument to control.tergm() (and have been renamed as the CMLE.form.ergm and CMLE.diss.ergm control arguments to control.stergm()). Furthermore, the arguments MCMC.init.maxedges and MCMC.init.maxchanges to control.stergm() have been replaced by the MCMC.maxedges and MCMC.maxchanges arguments to control.tergm(); these arguments have also been replaced in control.stergm(), so code continuing to use the old interface will still need to change from using MCMC.init.maxedges and MCMC.init.maxchanges to using MCMC.maxedges and MCMC.maxchanges.

Our discussion of initial coefficient values below will also include the necessary control argument changes for our examples above.

• The initial coefficient specifications for the formation and dissolution models, if passed, should be combined into a specification of initial coefficients for the combined model. This can be done through the tergm function's offset.coef, control$init, and/or control$CMLE.ergm$init arguments (the final one applying only to the CMLE case). If offset.coef is passed, it should have length equal to the number of offset thetas in the combined model, and if control$init or control$CMLE.ergm$init is passed, it should have length equal to the total number of thetas in the combined model. (NAs may be used in control$init or control$CMLE.ergm$init to indicate that initial values for those (non-offset) thetas are not being passed.) Here control refers to the control.tergm class control discussed in the previous bullet point.

In our examples, the CMLE call specifies initial coefficient values through control$CMLE.control.*$init. We can combine these into control$CMLE.ergm$init as

control = control.tergm(CMLE.ergm = control.ergm(init = c(-3.5,2,0,NA,0,1,0,1/2)))

noting that we also replaced control.stergm() with control.tergm(). We can simplify this further by exploiting new control list flattening features, writing

control = snctrl(init = c(-3.5,2,0,NA,0,1,0,1/2))

instead.

The EGMME call specifies only a single dissolution offset, which we can specify through offset.coef as

offset.coef = log(9)

Overall, this produces the new-style calls

data(samplk)
samp <- list(samplk1, samplk2, samplk3)
samp.fit <- tergm(samp ~ Form(~edges+mutual+cyclicalties+transitiveties) + 
                         Persist(~edges+mutual+cyclicalties+transitiveties),
                  estimate = "CMLE",
                  times = 1:3,
                  control = snctrl(init = c(-3.5,2,0,NA,0,1,0,1/2)))

for CMLE, and

data(florentine)
tergm.fit.1 <- tergm(flobusiness ~ Form(~edges+gwesp(0,fixed=T)) + 
                                   Persist(~offset(edges)),
                     targets = "formation",
                     offset.coef = log(9),
                     estimate = "EGMME",
                     control = control.tergm(SA.plot.progress=TRUE))

for EGMME.

Simulation

From a fitted tergm object

A call in tergm 3.x for simulating from a fitted stergm might look something like

stergm.sim.1 <- simulate(stergm.fit.1, 
                         stats.form = TRUE,
                         nsim = 1,
                         time.slices = 1000,
                         control = control.simulate.stergm(MCMC.init.maxchanges = 10000))

There is no simulate.stergm function in tergm 4.0, only a simulate.tergm function, so the changes described in this section are generally mandatory, with the exception of the control list class, which can be left as control.simulate.stergm if desired (although this is not recommended). Even if one calls the old stergm() function to estimate the model, calling simulate on the returned object will dispatch to the simulate.tergm function described here.

To convert from simulating a fitted stergm in tergm 3.x to simulating a fitted tergm in tergm 4.0, we make the following changes.

• Replace the coef.form and coef.diss arguments (which will default to the coefficients of the fitted stergm) with the coef argument (which will default to the coefficients of the fitted tergm), which is schematically of the form coef = c(coef.form, coef.diss), assuming the combined formula used when estimating the tergm was of the form described in the Estimation section (with Form(formation formula) preceding Persist(dissolution formula)).

These arguments are not passed in the example above, so no corresponding changes are needed in that example.

• Replace the stats.form and stats.diss arguments (if passed) with the stats argument, which will give all generative model statistics if set to TRUE.

In the example above, we pass stats.form = TRUE, so in the 4.0 version of the call, we will set stats = TRUE.

• The control argument (if passed), previously of class control.simulate.stergm, should be replaced by one of class control.simulate.tergm. This can be accomplished by replacing control.simulate.stergm() with control.simulate.tergm(), snctrl(), or list(), and updating arguments as follows. Arguments to control.simulate.stergm() occurring in pairs with .form and .diss in their names have been collapsed to single, correspondingly named arguments to control.simulate.tergm() without .form or .diss. Additionally, the arguments MCMC.init.maxedges and MCMC.init.maxchanges to control.simulate.stergm() have been replaced by the MCMC.maxedges and MCMC.maxchanges arguments to control.simulate.tergm(); these arguments have also been replaced in control.simulate.stergm(), so code continuing to use control.simulate.stergm() will still need to change from using MCMC.init.maxedges and MCMC.init.maxchanges to using MCMC.maxedges and MCMC.maxchanges.

In the example above, we passed MCMC.init.maxchanges = 10000; since this is enough to accomodate all expected changes throughout the entire simulation, we will pass

control = snctrl(MCMC.maxchanges = 10000)

in the 4.0 version of the call.

Thus, dropping the s from the object names for consistency, we obtain the 4.0 style call

tergm.sim.1 <- simulate(tergm.fit.1, 
                        stats = TRUE,
                        nsim = 1,
                        time.slices = 1000,
                        control = snctrl(MCMC.maxchanges = 10000))

From a network (or networkDynamic)

A call in tergm 3.x for simulating based on a starting network (or networkDynamic), along with specified formation and dissolution formulas and coefficients, might look something like

stergm.sim.2 <- simulate(flobusiness, 
                         formation = ~edges+gwesp(0,fixed=T),
                         dissolution = ~edges, 
                         monitor = "formation",
                         coef.form = c(-7.981749, 1.575780), 
                         coef.diss = log(99),
                         time.slices = 50000)

To convert from simulating based on a starting network in tergm 3.x to simulating based on a starting network in tergm 4.0, we make the following changes.

• Combine the network, formation, and dissolution formulas into a single formula, schematically of the form

network ~ Form(formation formula) + Persist(dissolution formula)

as for estimation.

• Combine the coef.form and coef.diss arguments into a single coef argument, schematically of the form coef = c(coef.form, coef.diss), assuming the combined formula is specified as in the previous bullet point (with Form(formation formula) preceding Persist(dissolution formula)).

• The control argument (if passed), previously of class control.simulate.network, should be replaced by one of class control.simulate.formula.tergm. This can be accomplished by replacing control.simulate.network() with control.simulate.formula.tergm(), snctrl(), or list(), and updating arguments as when simulating from a fitted tergm.

• Combine the stats.form and stats.diss arguments (if passed) into a single stats argument.

• Pass dynamic = TRUE to indicate that you want dynamic tergm simulation.

Thus, we obtain the 4.0 simulation call

tergm.sim.2 <- simulate(flobusiness ~ Form(~edges+gwesp(0,fixed=T)) +
                                      Persist(~edges),
                        monitor = "formation",
                        coef = c(-7.981749, 1.575780, log(99)), 
                        time.slices = 50000,
                        dynamic = TRUE)

statnet/tergm documentation built on Jan. 31, 2024, 12:10 p.m.