# 5. Spatial confounding and restricted spatial regression

When a covariate is itself spatially structured (a north–south gradient, deprivation, distance to a city), it is collinear with the spatial random effect: the data can be explained almost equally well by “covariate effect” or by “spatial noise”. This is spatial confounding. This tutorial is self-contained and deliberately balanced — spatial confounding is a genuinely nuanced topic.

## The issue

The model is \(\eta_i=d_i^\top\beta+S_i\) with \(S\sim\mathcal N(0,\sigma^2 R(\phi))\). When a column of \(d\) lies close to the leading spatial modes of \(R(\phi)\), the coefficient \(\beta\) and the random effect \(S\) compete to explain the same pattern. The consequence is that \(\widehat\beta\) is poorly identified — uncertain, and sensitive to how the spatial term is specified.

## Restricted spatial regression (RSR)

One remedy (Reich, Hodges & Zadnik 2006; Hughes & Haran 2013) constrains the random effect to the orthogonal complement of the fixed-effect design. With \(K\) an orthonormal basis of \(\mathrm{null}(D^\top)\), the model becomes \[ \eta=D\beta+K\alpha,\qquad \alpha\sim\mathcal N\!\big(0,\sigma^2 K^\top R(\phi)K\big), \] so \(K\alpha\) cannot reproduce any column of \(D\) and \(\beta\) is identified by the data, not by the spatial smoothing. `SDALGCP2` fits it by a Laplace-approximate marginal likelihood (derivation: `math/confounding-and-misalignment.pdf`).

    library(SDALGCP2)
    library(sf)

    set.seed(2)
    regions <- st_sf(geometry = st_make_grid(
      st_as_sfc(st_bbox(c(xmin = 0, ymin = 0, xmax = 20, ymax = 20))), n = c(8, 8)))
    N <- nrow(regions)
    pts <- sda_points(regions, delta = 1.2, method = 3)
    S   <- as.numeric(t(chol(0.7 * precompute_corr(pts, 4)$R[, , 1])) %*% rnorm(N))
    regions$x1    <- as.numeric(scale(st_coordinates(st_centroid(regions))[, 1]))  # spatial gradient
    regions$pop   <- round(runif(N, 500, 3000))
    regions$cases <- rpois(N, regions$pop * exp(-6 + 0.6 * regions$x1 + S))

    # standard fit vs restricted spatial regression
    fit_std <- sdalgcp(cases ~ x1 + offset(log(pop)), regions)
    fit_rsr <- sdalgcp(cases ~ x1 + offset(log(pop)), regions,
                       control = sdalgcp_control(confounding = "restricted"))

## What RSR does — and what it does not

Because the single-dataset estimate depends on the (random) alignment of the spatial field with the covariate, the honest way to see the effect is a small simulation study — 24 data sets generated as above (true \(\beta=0.6\)), fitting both methods:

    #> Across 24 simulated data sets (true beta = 0.6):
    #>   standard          mean est = 0.602,  SD of est = 0.265,  mean SE = 0.151
    #>   restricted (RSR)  mean est = 0.587,  SD of est = 0.267,  mean SE = 0.074

Two things stand out:

- both methods are essentially unbiased on average (0.60 vs 0.59) — here the spatial term is genuine noise, not a hidden confounder, so neither systematically attenuates \(\beta\);
- RSR reports a much smaller standard error (0.074 vs 0.151). Its estimate is identified purely by the within-covariate signal, so it looks very precise.

Caveat (important). That small RSR standard error can be anti-conservative: it does not reflect the across-data-set spread (\(\approx0.27\) here), because it treats the spatial–covariate separation as known. This is the well-documented critique of RSR (Hanks et al. 2015; Zimmerman & Ver Hoef 2021). Whether RSR helps depends on the causal role of the spatial term: if it is just smooth noise, RSR gives a clean, data-driven estimate; if it is a proxy for an unmeasured spatial confounder, the unrestricted model may actually be preferable.

## Practical guidance

- Use `confounding = "restricted"` when you want a coefficient identified by the covariate’s own variation, free of the spatial smoothing — but treat its standard error as a lower bound.
- When covariates are non-spatial (e.g. an individual-level rate), confounding is not a concern and the default fit is appropriate.
- In all cases, report which choice you made: the spatial term and the covariate are partly interchangeable, and that is a property of the data, not a bug.

## References

Reich, Hodges & Zadnik (2006) Biometrics; Hughes & Haran (2013) JRSS-B; Hanks, Schliep, Hooten & Hoeting (2015) Environmetrics; Zimmerman & Ver Hoef (2021) Spatial Statistics. ```