# Occupancy models with detection error

Let us continue with the simple occupancy model we used previously. Most applied ecologists are aware that the occupancy and abundance surveys have some level of detection error. Even if the species is present, for various reasons we may not observe its presence. Similarly we may not be able to count all the individuals that are present at a location. Let us look at how to model such a situation. We will discuss the model and then show how it can be looked upon as a hierarchical model.

### Notation

• $W_i$: this denotes the observed status at the location $i$, can be 0 or 1,
• $Y_i$: this denotes the true status at the location $i$, can be 0 or 1; this status is unknown.

### Assumptions

1. The observed status depends on the true status. If there is no dependence between the two variables, obviously we cannot do any inference.
2. There are no phantom’’ individuals. That is, if the true status is 0, we will observe 0 with probability 1.
3. True status at one location is independent of status of other locations.
4. Observation at one location is not affected by what we observed anywhere else (or, at other times at that location). Surveys are independent of each other.

We can extend the Bernoulli model from the introduction as follows:

• True status: $Y_i \sim Bernoulli(\varphi)$.
• Observed status: $(W_i \mid Y_i = y_i) \sim Bernoulli(p^{y_i} (1 - p)^{1 - y_i})$.

An important thing to note here is that we only observe $W$’s and not the true statuses ($Y$) which are unknown. We can use the standard probability rules to compute:

This is called the marginal distribution of $W$. We can write down the likelihood function as a function of parameters $(p, \varphi)$.

Cautionary note Just because one can write down the likelihood function, it does not mean one can estimate the parameters.

This is a simple situation with two parameters and hence we can plot the likelihood function as a contour plot.

R code for data generation:

set.seed(4321)
n <- 100
p <- 0.6
phi <- 0.4
y <- rbinom(n = n, size = 1, prob = phi)
w <- rbinom(n = n, size = y, prob = y * p)
table(Y = y, W = w)


Given the data, plot the likelihood contours.

## setting up the grid for p and phi
grid <- expand.grid(p = seq(0, 1, by = 0.01),
phi = seq(0, 1, by = 0.01),
L = NA)
## the likelihood function
L_fun <- function(w, p, phi) {
prod((p * phi)^w * (1 - p * phi)^(1 - w))
}
## calculating the likelihood for the grid
for (i in 1:nrow(grid)) {
grid$L[i] <- L_fun(w = w, p = grid$p[i], phi = grid$phi[i]) } ## plot the likelihood surface dcpal_reds <- colorRampPalette(c("#f9f2f4", "#c7254e")) L_mat <- matrix(grid$L, sqrt(nrow(grid)))
image(L_mat,
xlab = "p", ylab = expression(varphi),
col = dcpal_reds(12))
abline(h = phi, v = p, col = "#f9f2f4", lwd = 3)
abline(h = phi, v = p, col = "#c7254e", lwd = 1)
curve((p * phi) / x, 0, 1, add = TRUE,
col = "#18bc9c", lwd = 2)


We can see that the likelihood function looks like a mountain with a ridge tracing a curve corresponding to the product $p \varphi = c$.

library(rgl)
open3d()
bg3d("white")
material3d(col = "black")
dcpal_grbu <- colorRampPalette(c("#18bc9c", "#3498db"))
Col <- rev(dcpal_grbu(12))[cut(L_mat, breaks = 12)]
persp3d(L_mat / max(L_mat), col = Col,
theta=50, phi=25, expand=0.75, ticktype="detailed",
xlab = "p", ylab = "phi", zlab = "L")

• Likelihood function does not have a unique maximum. All values along this curve have equal support in the data. We can estimate the product but not the individual components of the product.
• The placement of the curve depends on the data. So there is information in the data only about the product but not the components.

When the likelihood function attains maximum at more than one parameter combination, we call the parameters non-estimable. There are various reasons for such non-estimability (Reference: Campbell and Lele, 2013 and a couple of references from that paper).

Structural problems with the model: it might be that the structure of the problem is such that no matter what, you cannot estimate the parameters. This is called non-identifiability.

Sometimes there are no structural issues but the observed data combination is such that the likelihood is problematic. This is called non-estimability. An example will be collinear covariates in regression.

Consequences of non-identifiability: management decisions can be based only on identifiable components of the model.

For models with more than two parameters, it is very difficult to plot the likelihood function. It is nearly impossible to diagnose non-identifiability and non-estimability of the parameters. Data cloning method provides a very simple approach to diagnose non-estimability for general hierarchical models.

We can skip all the mathematical details in the calculation of the likelihood function and use JAGS and MCMC to do almost all of the above analysis.

### Bayesian model in JAGS

library(dclone)
library(rjags)
library(lattice)
model <- custommodel("model {
for (i in 1:n) {
Y[i] ~ dbern(phi)
W[i] ~ dbern(Y[i] * p)
}
#p ~ dunif(0.001, 0.999) # alternative priors
#phi ~ dunif(0.001, 0.999)
p ~ dbeta(1, 1)
phi ~ dbeta(0.5, 0.5)
}")
dat <- list(W = w, n = n)

## try running this and see what happens:
#fit <- jags.fit(data = dat, params = c("p", "phi"),  model = model)
##  Error in node W[2]
##  Node inconsistent with parents

## ways of deining initial values
#ini <- list(Y = w)
ini <- list(Y = rep(1, n))
fit <- jags.fit(data = dat, params = c("p", "phi"),
model = model, init = ini)
summary(fit)
gelman.diag(fit)
plot(fit) # trace and density
pairs(fit) # bivariate density
densityplot(fit) # density by chains
qqmath(fit) # cumulative density by chains
xyplot(fit) # lattice based trace
acfplot(fit) # autocorrelation vs. lag
crosscorr.plot(fit) # correlation matrix image


• Traceplot and R-hat values indicate good mixing and convergence.
• Correlations are high, bivariate plots indicate problems.

### Data cloning

To make sure that both locations and clones are independent (i.i.d.), it is safest to include and extra dimension and the corresponding loop in the model.

library(dclone)
library(rjags)
model <- custommodel("model {
for (k in 1:K) {
for (i in 1:n) {
Y[i,k] ~ dbern(phi)
W[i,k] ~ dbern(Y[i,k] * p)
}
}
#p ~ dunif(0.001, 0.999) # alternative priors
#phi ~ dunif(0.001, 0.999)
p ~ dbeta(1, 1)
phi ~ dbeta(0.5, 0.5)
}")
dat <- list(W = dcdim(data.matrix(w)), n = n, K = 1)
ini <- list(Y = dcdim(data.matrix(w)))
## need to clone the initial values too
ifun <- function(model, n.clones) {
dclone(list(Y = dcdim(data.matrix(w))),
n.clones)
}
dcfit <- dc.fit(data = dat, params = c("p", "phi"),
model = model, inits = ini,
n.clones = c(1,2,4,8), unchanged = "n", multiply = "K",
initsfun = ifun, n.iter = 10000)
summary(dcfit)
plot(dcfit)
dctable(dcfit)
plot(dctable(dcfit))
dcdiag(dcfit)
plot(dcdiag(dcfit))
pairs(dcfit)


#### Modification

If locations are treated as i.i.d., it is possible to replicate the vector, so that length becomes n * K.

model <- custommodel("model {
for (i in 1:n) {
Y[i] ~ dbern(p)
W[i] ~ dbern(Y[i] * phi)
}
p ~ dunif(0.001, 0.999)
phi ~ dunif(0.001, 0.999)
}")
dat <- list(W = w, n = n)
ini <- list(Y = w)
ifun <- function(model, n.clones) {
dclone(list(Y = w), n.clones)
}
dcfit <- dc.fit(data = dat, params = c("p", "phi"),
model = model, inits = ini,
n.clones = c(1,2,4,8), multiply = "n",
initsfun = ifun)


• Traceplot and R-hat values indicate good mixing and convergence.
• Correlations are high, bivariate plots indicate problems.
• Observe what happens to the standard errors as we increase the number of clones. It does not converge to 0 as it did before. This indicates non-estimabilty of the parameters.
• lambda.max value is constant (not decreasing) with $K$, we’ll discuss why.

Suppose we go to the same location more than once, say $T$ times. Then sometimes we will observe the species and sometimes we will not. These changes may help us learn about the detection error process.

The occupancy model with replicate visits is:

• True status: $Y_i \sim Bernoulli(\varphi)$.
• Observed status: $(W_{i,t} \mid Y_i = 1) \sim Bernoulli(p)$ and $W_{i,t} \mid Y_i = 0$ equals 0 with probability 1.

The likelihood function is:

where $w_{i \cdot} = \sum^{t=1}{T} w{i,t}$ and $I( w_{i \cdot} = 0 )$ is an indicator function that is equal to one if $w_{i \cdot} = 0$.

### Assumptions

1. Closed population assumption: there is colonization or extinction, that is the true status remains the same over the visits.
2. Independent survey assumption: replicate visits are independent of each other.

R code for data generation:

set.seed(1234)
n <- 50
T <- 5
p <- 0.6
phi <- 0.4
y <- rbinom(n = n, size = 1, prob = phi)
w <- matrix(NA, n, T)
for (t in 1:T)
w[,t] <- rbinom(n = n, size = y, prob = y * p)


Given the data, plot the likelihood contours.

## setting up the grid for p and phi
grid <- expand.grid(p = seq(0, 1, by = 0.01),
phi = seq(0, 1, by = 0.01),
L = NA)
## the likelihood function
L_fun <- function(w, p, phi) {
wdot <- rowSums(w)
T <- ncol(w)
prod(phi * (choose(T, wdot) * p^wdot * (1 - p)^(T - wdot)) +
(1 - phi) * (wdot == 0))
}
## calculating the likelihood for the grid
for (i in 1:nrow(grid)) {
grid$L[i] <- L_fun(w = w, p = grid$p[i], phi = grid$phi[i]) } ## plot the likelihood surface dcpal_reds <- colorRampPalette(c("#f9f2f4", "#c7254e")) L_mat <- matrix(grid$L, sqrt(nrow(grid)))
image(L_mat,
xlab = "p", ylab = expression(varphi),
col = dcpal_reds(12))
abline(h = phi, v = p, col = "#f9f2f4", lwd = 3)
abline(h = phi, v = p, col = "#c7254e", lwd = 1)

library(rgl)
open3d()
bg3d("white")
material3d(col = "black")
dcpal_grbu <- colorRampPalette(c("#18bc9c", "#3498db"))
Col <- rev(dcpal_grbu(12))[cut(L_mat, breaks = 12)]
persp3d(L_mat / max(L_mat), col = Col,
theta=50, phi=25, expand=0.75, ticktype="detailed",
ylab = "p", xlab = "phi", zlab = "L")


### Bayesian model in JAGS

library(dclone)
library(rjags)
model <- custommodel("model {
for (i in 1:n) {
Y[i] ~ dbern(phi)
for (t in 1:T) {
W[i,t] ~ dbern(Y[i] * p)
}
}
p ~ dunif(0.001, 0.999)
phi ~ dunif(0.001, 0.999)
}")
dat <- list(W = w, n = n, T = T)
## initial values need to reflact realistic values
#ini <- list(Y = rep(1, nrow(w)))
ini <- list(Y = ifelse(rowSums(w) > 0, 1, 0))
fit <- jags.fit(data = dat, params = c("p", "phi"),
model = model, inits = ini)
summary(fit)
plot(fit)
pairs(fit)
crosscorr.plot(fit)


Much better right? Observe these:

• Good mixing, but now the mode is at the right values.
• No strong correlation.

### Bayesian inference

Effect of priors on the estimation and prediction of the occupancy proportion:

model <- custommodel("model {
for (i in 1:n) {
Y[i] ~ dbern(p)
for (t in 1:T) {
W[i,t] ~ dbern(Y[i] * phi)
}
}
p <- ilogit(logit_p)
phi <- ilogit(logit_phi)
logit_p ~ dnorm(-2, 1)
logit_phi ~ dnorm(2, 1)
}")
dat <- list(W = w, n = n, T = T)
ini <- list(Y = ifelse(rowSums(w) > 0, 1, 0))
fit2 <- jags.fit(data = dat, params = c("p", "phi"),
model = model, inits = ini)
summary(fit2)
plot(fit2)
pairs(fit2)


Compare posteriors based on the different priors: which one shall we prefer?

cbind(Truth=c(p, phi), Uniform_prior=coef(fit), Normal_prior=coef(fit2))


### Data cloning

Frequentist inference: Identifiability check, independence from the specification of the prior check, confidence intervals and predictions for the occupancy proportion.

library(dclone)
library(rjags)
model <- custommodel("model {
for (k in 1:K) {
for (i in 1:n) {
Y[i,k] ~ dbern(phi)
for (t in 1:T) {
W[i,t,k] ~ dbern(Y[i,k] * p)
}
}
}
p ~ dunif(0.001, 0.999)
phi ~ dunif(0.001, 0.999)
}")
dat <- list(W = dcdim(array(w, c(n,T,1))), n = n, T = T, K = 1)
ini <- list(Y = data.matrix(rep(1, n)))
ifun <- function(model, n.clones) {
list(Y = dclone(dcdim(data.matrix(rep(1, n))), n.clones))
}
dcfit <- dc.fit(data = dat, params = c("p", "phi"),
model = model, inits = ini,
n.clones = c(1,2,4,8), multiply = "K", unchanged = c("n","T"),
initsfun = ifun)
summary(dcfit)

## alternative prior specification
model <- custommodel("model {
for (k in 1:K) {
for (i in 1:n) {
Y[i,k] ~ dbern(phi)
for (t in 1:T) {
W[i,t,k] ~ dbern(Y[i,k] * p)
}
}
}
p <- ilogit(logit_p)
phi <- ilogit(logit_phi)
logit_p ~ dnorm(-2, 10)
logit_phi ~ dnorm(2, 10)
}")
dat <- list(W = dcdim(array(w, c(n,T,1))), n = n, T = T, K = 1)
ini <- list(Y = data.matrix(rep(1, n)))
ifun <- function(model, n.clones) {
list(Y = dclone(dcdim(data.matrix(rep(1, n))), n.clones))
}
dcfit2 <- dc.fit(data = dat, params = c("p", "phi"),
model = model, inits = ini,
n.clones = c(1,2,4,8), multiply = "K", unchanged = c("n","T"),
initsfun = ifun)
summary(dcfit2)

## no effect of prior
cbind(Truth=c(p, phi), Uniform_prior=coef(dcfit), Normal_prior=coef(dcfit2))

## see how prior effect is related to K
sapply(dctable(dcfit), "[[", "mean")
sapply(dctable(dcfit2), "[[", "mean")


### Generalization to take into account covariates

$p$ and $\varphi$ can be a function of independent variables with values varying across the $n$ location, for example:

• $p_i = \frac{exp(\theta_0 + \theta_1 z_i)}{1 + exp(\theta_0 + \theta_1 z_i)}$,
• $\varphi_i = \frac{exp(\beta_0 + \beta_1 x_i)}{1 + exp(\beta_0 + \beta_1 x_i)}$.

R code for data generation:

set.seed(1234)
n <- 1000
x <- rnorm(n)
z <- rnorm(n)
beta <- c(0, 1)
theta <- c(0.2, -0.5)
p <- exp(theta[1] + theta[2] * z) / (1 + exp(theta[1] + theta[2] * z))
phi <- exp(beta[1] + beta[2] * x) / (1 + exp(beta[1] + beta[2] * x))
#p <- plogis(model.matrix(~z) %*% theta)
#phi <- plogis(model.matrix(~x) %*% beta)
y <- rbinom(n = n, size = 1, prob = phi)
w <- rbinom(n = n, size = y, prob = p)
table(Y = y, W = w)

naive <- glm(w ~ x, family = binomial("logit"))
summary(naive)
library(detect)
m <- svocc(w ~ x | z)
summary(m)

model <- custommodel("model {
for (i in 1:n) {
W[i] ~ dbin(p[i] * phi[i], K)
logit(p[i]) <- inprod(Z[i,], theta)
logit(phi[i]) <- inprod(X[i,], beta)
}
beta[1] ~ dnorm(0, 0.001)
beta[2] ~ dnorm(0, 0.001)
theta[1] ~ dnorm(0, 0.001)
theta[2] ~ dnorm(0, 0.001)
}")
dat <- list(W = w, n = n, K = 1,
X = model.matrix(~x), Z = model.matrix(~z))
dcfit <- dc.fit(data = dat,
params = c("beta", "theta"), model = model,
n.clones = c(1, 10), n.iter = 2000,
unchanged = c("W", "n", "X", "Z"), multiply = "K")
summary(dcfit)
dctable(dcfit)
dcdiag(dcfit)
pairs(dcfit)


For a quasi-Bayesian approach, see here how to utilize the naive estimator to stabilize single visit based estimates:

model <- custommodel("model {
for (i in 1:n) {
W[i] ~ dbin(p[i] * phi[i], K)
logit(p[i]) <- inprod(Z[i,], theta)
logit(phi[i]) <- inprod(X[i,], beta)
}
beta[1] ~ dnorm(naive[1], penalty)
beta[2] ~ dnorm(naive[2], penalty)
theta[1] ~ dnorm(0, 0.001)
theta[2] ~ dnorm(0, 0.001)
}")
dat <- list(W = w, n = n, K = 1,
X = model.matrix(~x), Z = model.matrix(~z),
naive = coef(naive), penalty = 0.5)
dcfit <- dc.fit(data = dat,
params = c("beta", "theta"), model = model,
n.clones = c(1, 10),
n.update = 5000, n.iter = 2000,
unchanged = c("W", "n", "X", "Z", "naive", "penalty"),
multiply = "K")
summary(dcfit)
dctable(dcfit)
dcdiag(dcfit)
pairs(dcfit)