# Introduction to Durational Event Models

## Abstract

The redeem package provides tools for the estimation of Durational Event Models (DEM) via the `dem()` function. This framework extends standard Relational Event Models (REM), which can be fit using the `rem()` function, by explicitly modeling the duration of interactions. This vignette provides a theoretical overview of the model and demonstrates its application using a simulated dataset.

## Theoretical Framework

Unlike standard REMs that treat events as instantaneous, the DEM framework characterizes interactions via two separate intensities:

1. Incidence Process (\(\lambda^{0\rightarrow 1}\)): Models the intensity of a pair of actors \((i,j)\) to start an interaction.
2. Dissolution Process (\(\lambda^{1\rightarrow 0}\)): Models the intensity of a pair of actors \((i,j)\) to end an already started interaction.

The intensities are modeled as: \[\lambda_{ij}^{0\rightarrow 1}(t, \beta, \alpha, \gamma^{0\rightarrow 1}) = \exp(s_{i,j}(\mathscr{H}_t)^\top \beta + \alpha_i + \alpha_j + f(t, \gamma^{0\rightarrow 1}))\] \[\lambda_{ij}^{1\rightarrow 0}(t, \delta, \phi, \gamma^{1\rightarrow 0}) = \exp(u_{i,j}(\mathscr{H}_t)^\top \delta + \phi_i + \phi_j + f(t, \gamma^{1\rightarrow 0}))\]

where:

- \(\mathscr{H}_t\) defines the history of interactions until time \(t\).
- \(s_{i,j}(\mathscr{H}_t)\) and \(u_{i,j}(\mathscr{H}_t)\) are vectors of summary statistics.
- \(\alpha, \phi\) are actor-specific popularity parameters.
- \(f(t, \gamma) = \sum_{q=1}^Q \gamma_q \mathbb{I}(c_{q-1} \le t < c_q)\) is a baseline step-function that captures temporal variations in the data. The indicator function \(\mathbb{I}(c_{q-1} \le t < c_q)\) takes the value 1 if \(c_{q-1} \le t < c_q\) and 0 otherwise, where \(0 = c_0 < c_1 < \dots < c_Q\) are specified change points and \(\gamma = (\gamma_1, \dots, \gamma_Q)^\top\) is the baseline parameter vector (with \(\gamma_1 = 0\) imposed for model identifiability). Note that here we only look at undirected interactions, so the parameters are symmetric for \(i\) and \(j\). The model is also implemented for directed interactions allowing for asymmetric parameters, i.e., with \(\phi_{i,O}\) and \(\phi_{i,I}\) for the in and outgoing effect of actor \(i\) in the dissolution intensity.

## Summary Statistics

The package implements several key statistics to capture network dynamics. For full mathematical definitions and descriptions of the available transformations (e.g., `log`, `sig`, `bin`), please refer to the `redeem_terms` documentation.

Key terms include:

- Inertia: \(s_{ij}(\mathscr{H}_t) = N_{ij}(t)\), counting previous \(i \to j\) formation events.
- Degree Effects: Actor-specific activity (\(\alpha\)) and popularity (\(\phi\)).
- Duration: For the dissolution process, the elapsed time since the beginning of the interaction.
- Common Partners: Shared outgoing (OSP) or incoming (ISP) friends.

## Fine-tuning with

The estimation process can be customized using the function. This function creates a configuration object that manages algorithmic behavior:

- `estimate`: Character; estimation method for (“Blockwise” or “NR”). “Blockwise” is the default and recommended for larger networks.
- `it_max` & `tol`: Integer and Numeric; maximum number of iterations and convergence tolerance. These can be given as a vector of length 2 to specify different limits for the formation (first element) and dissolution (second element) processes in .
- `accelerated`: Logical; if `TRUE`, uses SQUAREM acceleration for MM updates to speed up convergence. Can also be a vector of length 2 for process-specific acceleration.
- `verbose`: Logical; if `TRUE`, prints iteration-specific progress.
- `simultaneous_interactions`: Logical; controls whether actors can participate in multiple interactions at once.
- `return_data`: Logical; whether to include preprocessed data frames in the result.

For example, to use more iterations for the dissolution process:

    ctrl <- control.redeem(it_max = c(50, 200), tol = c(1e-8, 1e-12))

## Example: Simulating and Fitting a DEM

    library(redeem)

### Data Preparation

A DEM event sequence contains durations encoded sequentially via interaction states. The event matrix demands four columns: `time`, `from`, `to`, and `type`. A `type = 1` transitions the dyad into an active interaction, while `type = 0` dissolves it.

    # Simulated continuous-duration interaction sequence
    n_nodes <- 10
    events <- matrix(c(
      1.0, 1, 2, 1, # Node 1 initiates tie with 2
      1.5, 3, 4, 1, # Node 3 initiates tie with 4
      2.0, 1, 2, 0, # Tie (1,2) concludes (duration 1.0)
      2.8, 3, 4, 0, # Tie (3,4) concludes (duration 1.3)
      3.5, 1, 3, 1, # Node 1 initiates tie with 3
      4.0, 1, 3, 0 # Tie (1,3) concludes (duration 0.5)
    ), ncol = 4, byrow = TRUE)
    colnames(events) <- c("time", "from", "to", "type")

### Model Fitting

To estimate a DEM, utilize the `dem()` function, detailing unique structural formulas for the onset (`formula_0_1`) and offset (`formula_1_0`) transition states. These formulas are specified using the model terms documented in `redeem_terms`.

    # Fit the Durational Event Model
    fit_dem <- dem(
      events = events,
      n_nodes = n_nodes,
      formula_0_1 = ~1, # Predictors for tie onset
      formula_1_0 = ~1, # Predictors for tie offset
      control = control.redeem(estimate = "Blockwise")
    )

    # View summaries using `summary.redeem_result`
    summary(fit_dem)
    #> Call:
    #> dem(events = events, formula_0_1 = ~1, formula_1_0 = ~1, n_nodes = n_nodes,
    #>     control = control.redeem(estimate = "Blockwise"))
    #>
    #> Results for Incidence Intensity (0 -> 1):
    #> Fixed Effects:
    #>           Estimate Std. Error t value  Pr(>|t|)
    #> Intercept -4.07867    0.57734 -7.0646 1.611e-12 ***
    #> ---
    #> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
    #>
    #> Log-likelihood: -15.236
    #>
    #> Results for Duration Intensity (1 -> 0):
    #> Fixed Effects:
    #>           Estimate Std. Error t value Pr(>|t|)
    #> Intercept 0.068993   0.577349  0.1195   0.9049
    #>
    #> Log-likelihood: -2.793
    #>
    #> Combined Model Fit:
    #>   AIC: 40.05804
    #>   BIC: 40.45249
    #>
    #> Total estimation time: 0.008709908 secs

### Interpretation

- \(\beta\) (Incidence): Positive parameters reflect an augmented intensity to create a network tie.
- \(\delta\) (Dissolution): Positive parameters indicate an accelerated intensity to break a network tie (leading to shorter average interaction durations), while negative parameters identify ties structured to last considerably longer.

## Simulation and Model Diagnostics

Evaluating the final DEM allows you to guarantee that your dynamic formulas correctly abstract the empirical interaction realities.

### Predicting and Simulating Events

Because the DEM explicitly represents the generative continuous-time processes, calling the `simulate()` method synthesizes completely new behavioral pathways extrapolated from the estimated parameters.

    # Simulate networks matching the observed bounds
    simulated_events <- simulate(fit_dem, nsim = 100, time_horizon = 10)

### Residual Analysis

To verify individual probabilistic predictions along the network evolution, analyze the estimated intensity trajectories via Cox-Snell residuals.

Upon formulating the predicted cumulative intensities up to the terminal interaction lengths using the `predict()` method, the expected cumulative intensities strictly conform to an \(Exp(1)\) exponential distribution if perfectly calibrated.

The redeem package provides the `get_residuals()` function to automate this check. It calculates the cumulative intensities for all dyads and returns Kaplan-Meier estimates of the residual survival function alongside the theoretical \(Exp(1)\) curve.

    # Extract residuals for diagnostics using `get_residuals()`
    # Note: Ensure return_data = TRUE was set in `control.redeem()`
    resids <- get_residuals(fit_dem)

    # Plot the Kaplan-Meier estimate of the residual survival vs. Theoretical Exp(1)
    plot(resids$time, resids$surv,
      type = "l", log = "y",
      xlab = "Cox-Snell Residuals", ylab = "Survival Probability",
      main = "Cox-Snell Residual Diagnostic"
    )
    #> Warning in xy.coords(x, y, xlabel, ylabel, log): 1 y value <= 0 omitted from
    #> logarithmic plot
    lines(resids$time, resids$theoretical, col = "red", lty = 2)
    legend("topright",
      legend = c("Empirical", "Theoretical Exp(1)"),
      col = c("black", "red"), lty = 1:2
    )

If the model is accurately specified, the empirical survival curve (black line) should closely follow the theoretical exponential decay (red dashed line). Significant deviations, especially in the tails, suggest that the model fails to capture certain temporal dynamics or that there is unmodeled heterogeneity among dyads.

## References

Fritz, C., Rastelli, R., Fop, M., & Caimo, A. (2026). Scalable Durational Event Models: Application to Physical and Digital Interactions. arXiv preprint arXiv:2504.00049.