# plotting interaction effects

    library(modsem)

# Plotting Interaction Effects

Interaction effects can be plotted using the included `plot_interaction()` function. This function takes a fitted model object and the names of the two variables that are interacting. The function will plot the interaction effect of the two variables, where:

- The x-variable is plotted on the x-axis.
- The y-variable is plotted on the y-axis.
- The z-variable determines at which points the effect of x on y is plotted.

The function will also plot the 95% confidence interval for the interaction effect. Note that the `vals_z` argument (as well as the values of `x`) are scaled by the mean and standard deviation of the variables. Unless the `rescale` argument is set to `FALSE`.

Here is a simple example using the double-centering approach:

    m1 <- "
    # Outer Model
      X =~ x1 + x2 + x3
      Z =~ z1 + z2 + z3
      Y =~ y1 + y2 + y3

    # Inner Model
      Y ~ X + Z + X:Z
    "
    est1 <- modsem(m1, data = oneInt)
    plot_interaction(x = "X", z = "Z", y = "Y",
                     vals_z = -3:3, model = est1)

Here is a different example using the `lms` approach in the theory of planned behavior model:

    tpb <- "
    # Outer Model (Based on Hagger et al., 2007)
      ATT =~ att1 + att2 + att3 + att4 + att5
      SN =~ sn1 + sn2
      PBC =~ pbc1 + pbc2 + pbc3
      INT =~ int1 + int2 + int3
      BEH =~ b1 + b2

    # Inner Model (Based on Steinmetz et al., 2011)
      INT ~ ATT + SN + PBC
      BEH ~ INT + PBC
      BEH ~ PBC:INT
    "

    est2 <- modsem(tpb, TPB, method = "lms")
    #> Warning: It is recommended that you have at least 32 nodes for interaction
    #> effects between exogenous and endogenous variables in the lms approach 'nodes =
    #> 24'
    plot_interaction(x = "INT", z = "PBC", y = "BEH",
                     vals_z = c(-0.5, 0.5), model = est2)

# Plotting Johnson-Neyman Regions

The `plot_jn()` function can be used to plot Johnson-Neyman regions for a given interaction effect. This function takes a fitted model object, the names of the two variables that are interacting, and the name of the interaction effect. The function will plot the Johnson-Neyman regions for the interaction effect.

The `plot_jn()` function will also plot the 95% confidence interval for the interaction effect.

`x` is the name of the x-variable, `z` is the name of the z-variable, and `y` is the name of the y-variable. `model` is the fitted model object. The argument `min_z` and `max_z` are used to specify the range of values for the moderating variable.

Here is an example using the `ca` approach in the Holzinger-Swineford (1939) dataset:

    m1 <-  '
      visual  =~ x1 + x2 + x3
      textual =~ x4 + x5 + x6
      speed   =~ x7 + x8 + x9

      visual ~ speed + textual + speed:textual
    '

    est1 <- modsem(m1, data = lavaan::HolzingerSwineford1939, method = "ca")
    plot_jn(x = "speed", z = "textual", y = "visual", model = est1, max_z = 6)

Here is another example using the `qml` approach in the theory of planned behavior model:

    tpb <- "
    # Outer Model (Based on Hagger et al., 2007)
      ATT =~ att1 + att2 + att3 + att4 + att5
      SN =~ sn1 + sn2
      PBC =~ pbc1 + pbc2 + pbc3
      INT =~ int1 + int2 + int3
      BEH =~ b1 + b2

    # Inner Model (Based on Steinmetz et al., 2011)
      INT ~ ATT + SN + PBC
      BEH ~ INT + PBC
      BEH ~ PBC:INT
    "

    est2 <- modsem(tpb, TPB, method = "qml")
    plot_jn(x = "INT", z = "PBC", y = "BEH", model = est2,
            min_z = -1.5, max_z = -0.5)
    #> Warning: Truncating SD-range on the right and left!

# Plotting (3D) Surface Plots

The `plot_surface()` function can be used to plot 3D surface plots for a given interaction effect. This function takes a fitted model object, the names of the two variables that are interacting, and the name of the dependent variable. The function will plot the 3D surface plot for the interaction effect.

    tpb <- "
    # Outer Model (Based on Hagger et al., 2007)
      ATT =~ att1 + att2 + att3 + att4 + att5
      SN =~ sn1 + sn2
      PBC =~ pbc1 + pbc2 + pbc3
      INT =~ int1 + int2 + int3
      BEH =~ b1 + b2

    # Inner Model (Based on Steinmetz et al., 2011)
      INT ~ ATT + SN + PBC
      BEH ~ INT + PBC
      BEH ~ PBC:INT
    "

    est2 <- modsem(tpb, TPB, method = "qml")
    plot_surface(x = "INT", z = "PBC", y = "BEH", model = est2)

# Quadratic Effects and Response Surface Models

As of version `1.0.13`, `modsem` also handles quadratic effects when plotting interaction using the `plot_interaction()` and `plot_surface()` functions. Here we can see an example using some generated data for a model with two quadratic effects, and a single interaction effect. The model is equivalent to a typical response surface regression.

## Generating Data

    set.seed(2934269)

    var_X      <- 1
    var_Z      <- 1
    cov_X_Z    <- 0.2

    zeta_Y     <- 0.4

    gamma_Y_X  <-  0
    gamma_Y_Z  <-  0
    gamma_Y_XZ <-  1
    gamma_Y_ZZ <-  3 # exclude for now
    gamma_Y_XX <- -1 # exclude for now

    lambda_1   <- 1
    lambda_2   <- .7
    lambda_3   <- .8

    epsilon    <- 0.2
    beta_1     <- 1.2
    beta_2     <- 0.8
    beta_3     <- 1.5
    N          <- 1500

    residual <- \(epsilon) rnorm(N, sd = sqrt(epsilon))
    create_ind <- \(lv, beta, lambda, epsilon) beta + lambda * lv + residual(epsilon)

    Phi <- matrix(c(var_X, cov_X_Z,
                    cov_X_Z, var_Z), nrow = 2)
    XI <- mvtnorm::rmvnorm(N, sigma = Phi)

    X <- XI[, 1]
    Z <- XI[, 2]

    Y <-
      gamma_Y_X * X +
      gamma_Y_Z * Z +
      gamma_Y_XZ * X * Z +
      gamma_Y_XX * X * X +
      gamma_Y_ZZ * Z * Z +
      residual(zeta_Y)

    x1 <- create_ind(X, beta_1, lambda_1, epsilon)
    x2 <- create_ind(X, beta_2, lambda_2, epsilon)
    x3 <- create_ind(X, beta_3, lambda_2, epsilon)

    z1 <- create_ind(Z, beta_1, lambda_1, epsilon)
    z2 <- create_ind(Z, beta_2, lambda_2, epsilon)
    z3 <- create_ind(Z, beta_3, lambda_2, epsilon)

    y1 <- create_ind(Y, beta_1, lambda_1, epsilon)
    y2 <- create_ind(Y, beta_2, lambda_2, epsilon)
    y3 <- create_ind(Y, beta_2, lambda_2, epsilon)

    data.sim <- data.frame(
       x1, x2, x3,
       z1, z2, z3,
       y1, y2, y3
    )

## Fitting Model

Here we fit the model using the QML approach.

    model <- '
      X =~ x1 + x2 + x3
      Z =~ z1 + z2 + z3
      Y =~ y1 + y2 + y3

      Y ~ X + Z + X:X + Z:Z + X:Z
    '

    est3 <- modsem(model, data = data.sim, method = "qml")

## 2D Plot

Here we can see a two dimensional plot, where the slope of `X` no longer is constant at a given value of `Z`, since `X` has an interaction effect with itself.

    plot_interaction(x = "X", z = "Z", y = "Y", model = est3, vals_z = c(-1, 0, 1))

## 3D (Response Surface) Plot

Here we can see a three-dimensional surface plot.

    plot_surface(x = "X", z = "Z", y = "Y", model = est3)