# remstats

    library(remstats)

## Introduction

Relational event modeling approaches enable researchers to investigate both exogenous and endogenous factors influencing the evolution of a time-ordered sequence of relational events - commonly known as a relational event history. These models are categorized into tie-oriented models, where the probability of a dyad interacting next is modeled in a single step (e.g., see Butts, 2008), and actor-oriented models, which first model the probability of a sender initiating an interaction and subsequently the probability of the senders’ choice of receiver (e.g., see Stadtfeld & Block, 2017). The `R` package `remstats` is designed to compute a variety of statistics for both types of models.

The `remstats` package is part of a bundle of `R`-packages developed by researchers from Tilburg University intended to aid applied researchers in the application of relational event modeling. For preparing the relational event history, `remstats` assumes the prior application of `remify::remify()` (available on CRAN). Model estimation can subsequently be executed using `remstimate` (available on GitHub at github.com/TilburgNetworkGroup/remstimate).

The following provides a brief introduction to computing statistics for relational event history data with remstats. We’ll begin with a quick workflow example, followed by a detailed description of the procedure for the tie-oriented model. Finally, we’ll provide an explanation of the procedure for the actor-oriented model.

## Workflow example

    # Load data
    data(history)
    data(info)

    # Define effects
    effects <- ~ 1 + send("extraversion", info) + inertia()

    # Prepare event history with the 'remify package'
    rehObject <- remify::remify(edgelist = history, model = "tie")

    # Compute statistics
    statsObject <- remstats(reh = rehObject, tie_effects = effects)

    # Estimate model parameters with the 'remstimate' package
    # fit <- remstimate::remstimate(reh = rehObject, stats = statsObject,
    #   method = "MLE", timing = "interval")

## Getting started

### Data

Relational event history data describes a time-ordered series of interactions between actors in a network. Such interactions are referred to as relational events. A relational event minimally contains information on the time of the event and the actors that are involved in the event.

As an illustration, we use the `history` data object in the `remstats` package. This data object is a randomly generated relational event history. A description of the simulated data can be accessed with `?history`. Here, we read that `history` is a small simulated relational event history with 115 events. Besides information on the time and actors, for each event there is also information on the setting and an event weight. We can view the first six events with:

    head(history)
    #>   time actor1 actor2 setting weight
    #> 1  238    105    113    work   1.33
    #> 2  317    105    109    work   1.64
    #> 3  345    115    112    work   1.82
    #> 4  627    101    115  social   1.25
    #> 5  832    113    107  social   1.67
    #> 6  842    105    109    work   2.30

We prepare the relational event history for computation of statistics for the tie-oriented model with the `remify` function from the `remify` package. Whenever the weight variable is present in the `edgelist` supplied to `remify`, it assumes that we want to use these to weight the events in the computation of the statistics. In this example, we don’t want this, thus we set the weight to one for all events:

    history$weight <- 1
    reh <- remify::remify(edgelist = history, model = "tie")

Besides the relational event history itself, relational event modeling often requires a second data object with exogenous information on the actors in the network. Information on the actors in the simulated data example in `remstats` is stored in the `info` object. A description of the `info` data can be accessed with `?info`. Here, we read that the `info` object stores for the 10 actors in the network information on their age, sex, extraversion and agreeableness score. Moreover, extraversion and agreeableness is measured multiple times during the observation period. The time variable tells us when the values change. We can view the attribute information for the first two actors with:

    head(info)
    #>   name  time age sex extraversion agreeableness
    #> 1  101     0   0   0        -0.40         -0.14
    #> 2  101  9432   0   0        -0.32         -0.26
    #> 3  101 18864   0   0        -0.53         -0.45
    #> 4  103     0   0   0        -0.13         -0.65
    #> 5  103  9432   0   0        -0.43         -0.44
    #> 6  103 18864   0   0        -0.13         -0.43

### Compute statistics for the tie-oriented model

First, we compute statistics for modeling relational event history data with a tie-oriented model. The statistics that are requested need to be supplied to the `tie_effects` argument of `remstats()`, specified in an object of class `formula`. This specification should be in the form `~ terms`.

An overview of the statistics that can be computed for the tie-oriented model is available using the `tie_effects()` function or its help documentation `?tie_effects`:

In this illustration, we start with requesting only one statistic: the inertia statistic. Most statistics can be tailored to the user’s needs. For example, lets view the description for the `inertia` statistic using `?inertia`. Here, we can read that the inertia statistic computes for every timepoint t for every pair of actors (i,j) in the riskset the number of past events. With the `scaling` argument, one of the methods for scaling the statistic can be chosen. The `consider_type` argument is relevant when event types are in the dependent variable, which we do not consider in this example.

In this illustration, we will standardize the inertia statistic. To request this statistic, we define the formula as follows:s

    effects <- ~ inertia(scaling = "std")

Now, we have everything we need to compute our first statistic:

    out <- remstats(tie_effects = effects, reh = reh)

The `remstats()` function outputs a 3-dimensional array with statistics for the tie-oriented model. On the rows of this array are the timepoints, the columns refer to the potential events in the riskset and the slices refer to the different statistics:

    dim(out)
    #> [1] 115  90   2

Our statistics object has 115 rows, corresponding to the 115 time points in the relational event history. It has 90 columns, corresponding to the 90 events in the riskset. The statistics object has two slices, that is because the baseline statistics is automatically computed when the timing of the events in the relational event history is exact (unless removed by specifying `-1` in the `effects` formula) and saved in the first slice. The `remstats()` procedure assumes that the timing of the events in the relational event history is exact and the full likelihood is used in the estimation, unless the argument `ordinal` in `remify::remify()` is set to `TRUE`.

We can view the names of the statistics that are in the statistics object with:

    out
    #> Relational Event Network Statistics
    #> > Model: tie-oriented
    #> > Computation method: per time point
    #> > Dimensions: 115 time points x 90 dyads x 2 statistics
    #> > Statistics:
    #>   >> 1: baseline
    #>   >> 2: inertia

Here, we see that, indeed, a baseline and inertia statistic are computed.

Since we did not request anything special for the riskset in `remify::remify()`, it consists of every directed pair of actors observed in the relational event history, which is 10*9 = 90 pairs. These pairs are saved in the `riskset` attribute. We can ask for the first few lines of this riskset:

    head(attr(out, "riskset"))
    #>   sender receiver id
    #> 1    101      103  1
    #> 2    101      104  2
    #> 3    101      105  3
    #> 4    101      107  4
    #> 5    101      109  5
    #> 6    101      111  6

Here, we see that the first event in the riskset is the event were actor 101 sends an interaction directed towards actor 103. The id column refers to the column in the statistic object that contains the statistic(s) for this specific dyad. The first column in the statistic object refers to this first event in the riskset, the second column in the statistic object to the second event in the riskset, and so forth.

Inertia is an example of an endogenous statistic: it is a function of the relational event history itself. Next, we are going to add a request for an exogenous statistic. For this we need the exogenous information on the actors in the `info` object.

As an illustration, we are going to request the statistic for an effect of extraversion on sending events, i.e., a send effect. The description of a send effect is accessed with `?send`. Here, we read that we need to supply the variable for which we want to specify a sender effect and that this variable should correspond to a column in the `attr_actors` object that we supply. Thus, we specify a send effect of extraversion with `send("extraversion", attr_actors = info)`. Here, we specify the `attr_actors` object within the `send()` function. Alternatively, it can be supplied to `remstats()`. This is for example useful if you want to compute a bunch of exogenous statistics using the same `attr_actors` object.

Statistics in the `effects` formula should be separated with the `+`. Finally, we add an interaction between the `inertia()` statistic and the `send()` statistic. This can be done by using the `*` or `:` operator:

    effects <- ~ inertia(scaling = "std") + send("extraversion", info) +
        inertia(scaling = "std"):send("extraversion", info)
    out <- remstats(tie_effects = effects, reh = reh)

### Compute statistics for the actor-oriented model

The procedure to compute statistics for the actor-oriented model is largely similar to what is written above, except that statistics have to be specified separately for the sender activity rate step of the model and the receiver choice step of the model. The statistics requested for these two modeling steps need to be supplied to two different effects arguments, namely `sender_effects` and `receiver_effects`, respectively.

An overview of the statistics that are available for the actor-oriented model in the two modeling steps can be obtained using the `actor_effects()` function or its help documentation `?actor_effects`.

In this illustration, we start with requesting only one statistic for the sender activity rate step: the outdegreeSender statistic. First, lets view the description for the `outdegreeSender` statistic using `?outdegreeSender`. Here, we can read that, in the sender activity rate step of the actor-oriented model, the outdegreeSender statistic computes for every timepoint t for every actors i the number of outgoing past events. With the `scaling` argument, one of the methods for scaling the statistic can be chosen.

First, we prepare the event history for computing statistics for an actor-oriented model:

    reh <- remify::remify(edgelist = history, model = "actor")

To compute the outdegreeSender statistic for the sender activity rate step we supply it to the `sender_effects` argument of `remstats()`:

    effects <- ~ outdegreeSender()
    out <- remstats(sender_effects = effects, reh = reh)

The outputted remstats object is now a list with two elements: `sender_stats` and `receiver_stats`:

    names(out)
    #> [1] "sender_stats"   "receiver_stats"

Since we did not request any statistics for the receiver choice step here, the `receiver_stats` object is empty. The `sender_stats` object contains the statistic array with the `baseline` statistic (again, automatically computed unless `ordinal = TRUE`), and the requested `outdegreeSender` statistic:

    out
    #> Relational Event Network Statistics
    #> > Model: actor-oriented
    #> > Computation method: per time point
    #> > Sender model:
    #>   >> Dimensions: 115 time points x 10 actors x 2 statistics
    #>   >> Statistics:
    #>       >>> 1: baseline
    #>       >>> 2: outdegreeSender

The dimension of `out$sender_stats` is 115 x 10 x 2. On the rows we have the timepoints, the columns refer to the actors that can be senders and the slices refer to the different statistics.

Lets extend our model and also request a statistic for the receiver choice step:

    sender_effects <- ~ outdegreeSender()
    receiver_effects <- ~ inertia()
    out <- remstats(sender_effects = sender_effects, receiver_effects = receiver_effects, reh = reh)

We can access the statistic computed for the receiver choice step with `out$receiver_stats`. In this step, the baseline statistic is not automatically computed (and not defined). Hence, the dimensions of the statistics object for the receiver choice step are 115 x 10 x 1. On the rows we have again the timepoints, on the columns now the receivers and on the slices the statistics.

Note that the computed values of the statistic in the receiver choice step are equal to the values for this receiver, given the current sender. For example, lets review the first six lines:

    # Set the column names equal to the receivers
    colnames(out$receiver_stats) <- attributes(reh)$dictionary$actors$actorName
    # Set the rownames equal to the senders
    rownames(out$receiver_stats) <- reh$edgelist$actor1
    # View the first six lines
    head(out$receiver_stats[,,"inertia"])
    #>     101 103 104 105 107 109 111 112 113 115
    #> 105   0   0   0   0   0   0   0   0   0   0
    #> 105   0   0   0   0   0   0   0   0   1   0
    #> 115   0   0   0   0   0   0   0   0   0   0
    #> 101   0   0   0   0   0   0   0   0   0   0
    #> 113   0   0   0   0   0   0   0   0   0   0
    #> 105   0   0   0   0   0   1   0   0   1   0

At the first timepoint, the inertia statistic for all receivers given the current sender (actor 105) is zero because no prior events have occurred. At the second timepoint, the sender is again actor 105. Now the inertia statistic is equal to the 1 for the receiver of the first event (actor 113). At the third timepoint, the inertia statistic is again zero for all receivers because now the sending actor is 115, who did not send any prior events.