PRISM plot (Partial Residual Intervenor Scatterplot Matrix)

Creates a matrix of scatterplots for diagnosing antedependence structure. The upper triangle shows ordinary scatterplots of Y[i] vs Y[j]. The lower triangle shows PRISM plots: residuals from regressing Y[i] and Y[j] on the intervenor variables Y[i+1], ..., Y[j-1].

Usage

plot_prism(
  y,
  time_labels = NULL,
  pch = 20,
  cex = 0.6,
  col_upper = "steelblue",
  col_lower = "firebrick",
  main = "PRISM Diagnostic Plot"
)

Arguments

y: Numeric matrix with n_subjects rows and n_time columns.
time_labels: Optional character vector of time point labels. Default uses column names or "T1", "T2", etc.
pch: Point character for scatterplots. Default 20 (filled circle).
cex: Point size. Default 0.6.
col_upper: Color for upper triangle plots. Default "steelblue".
col_lower: Color for lower triangle (PRISM) plots. Default "firebrick".
main: Overall title. Default "PRISM Diagnostic Plot".

Value

Invisibly returns NULL. Called for side effect (plotting).

Details

Under an antedependence model of order p, the partial correlation between Y[i] and Y[j] given the intervenors should be zero when |i-j| > p. This means PRISM plots in the lower triangle should show no association for lags greater than p.

Interpretation:

Upper triangle: Shows marginal associations between time points
Lower triangle (PRISM): Shows conditional associations after removing effects of intervenor variables
If AD(1) holds: Only the first sub-diagonal of lower triangle should show association
If AD(2) holds: First two sub-diagonals should show association

References

Zimmerman, D. L. and Nunez-Anton, V. (2009). Antedependence Models for Longitudinal Data. CRC Press. Chapter 2.

Examples

if (FALSE) { # \dontrun{
data("bolus_inad")
plot_prism(bolus_inad$y)

# With custom labels
plot_prism(bolus_inad$y, time_labels = paste0("Hour ", seq(0, 44, by = 4)))
} # }