arm (version 1.4-13)

sim: Functions to Get Posterior Distributions

Description

This generic function gets posterior simulations of sigma and beta from a lm object, or simulations of beta from a glm object, or simulations of beta from a mer object

Usage

sim(object, ...)

## S3 method for class 'lm':
sim(object, n.sims = 100)
## S3 method for class 'glm':
sim(object, n.sims = 100)
## S3 method for class 'polr':
sim(object, n.sims = 100)
## S3 method for class 'mer':
sim(object, n.sims = 100)

## S3 method for class 'sim':
coef(object)
## S3 method for class 'sim.polr':
coef(object, slot=c("ALL", "coef", "zeta"))
## S3 method for class 'sim.mer':
coef(object)
## S3 method for class 'sim.mer':
fixef(object)
## S3 method for class 'sim.mer':
ranef(object)

Arguments

object

the output of a call to lm with n data points and k predictors.

slot

return which slot of sim.polr, available options are coef, zeta, ALL.

...

further arguments passed to or from other methods.

n.sims

number of independent simulation draws to create.

Value

• coef
  matrix (dimensions n.sims x k) of n.sims random draws of coefficients.
• zeta
  matrix (dimensions n.sims x k) of n.sims random draws of zetas (cut points in polr).
• fixef
  matrix (dimensions n.sims x k) of n.sims random draws of coefficients of the fixed effects for the mer objects. Previously, it is called unmodeled.
• sigma
  vector of n.sims random draws of sigma (for glm's, this just returns a vector of 1's or else of the square root of the overdispersion parameter if that is in the model)

References

Andrew Gelman and Jennifer Hill. (2006). Data Analysis Using Regression and Multilevel/Hierarchical Models. Cambridge University Press.

See Also

display, lm, glm, lmer

Examples

#Examples of "sim" 
 set.seed (1)
 J <- 15
 n <- J*(J+1)/2
 group <- rep (1:J, 1:J)
 mu.a <- 5
 sigma.a <- 2
 a <- rnorm (J, mu.a, sigma.a)
 b <- -3
 x <- rnorm (n, 2, 1)
 sigma.y <- 6
 y <- rnorm (n, a[group] + b*x, sigma.y)
 u <- runif (J, 0, 3)
 y123.dat <- cbind (y, x, group)
# Linear regression 
 x1 <- y123.dat[,2]
 y1 <- y123.dat[,1]
 M1 <- lm (y1 ~ x1)
 display(M1)
 M1.sim <- sim(M1)
 coef.M1.sim <- coef(M1.sim)
 sigma.M1.sim <- sigma.hat(M1.sim)
 ## to get the uncertainty for the simulated estimates
 apply(coef(M1.sim), 2, quantile)
 quantile(sigma.hat(M1.sim))
 
# Logistic regression 
 u.data <- cbind (1:J, u)
 dimnames(u.data)[[2]] <- c("group", "u")
 u.dat <- as.data.frame (u.data)
 y <- rbinom (n, 1, invlogit (a[group] + b*x))
 M2 <- glm (y ~ x, family=binomial(link="logit"))
 display(M2)
 M2.sim <- sim (M2)
 coef.M2.sim <- coef(M2.sim)
 sigma.M2.sim <- sigma.hat(M2.sim)

# Ordered Logistic regression 
house.plr <- polr(Sat ~ Infl + Type + Cont, weights = Freq, data = housing)
display(house.plr)
M.plr <- sim(house.plr)
coef.sim <- coef(M.plr, slot="coef")   
zeta.sim <- coef(M.plr, slot="zeta")
coefall.sim <- coef(M.plr)

# Using lmer:
# Example 1
 E1 <- lmer (y ~ x + (1 | group))
 display(E1)
 E1.sim <- sim (E1)
 coef.E1.sim <- coef(E1.sim)
 fixef.E1.sim <- fixef(E1.sim)
 ranef.E1.sim <- ranef(E1.sim)
 sigma.E1.sim <- sigma.hat(E1.sim)


# Example 2
 u.full <- u[group]
 E2 <- lmer (y ~ x + u.full + (1 | group))
 display(E2)
 E2.sim <- sim (E2)
 coef.E2.sim <- coef(E2.sim)
 fixef.E2.sim <- fixef(E2.sim)
 ranef.E2.sim <- ranef(E2.sim)
 sigma.E2.sim <- sigma.hat(E2.sim)


# Example 3 
 y <- rbinom (n, 1, invlogit (a[group] + b*x))
 E3 <- glmer (y ~ x + (1 | group), family=binomial(link="logit"))
 display(E3)
 E3.sim <- sim (E3)
 coef.E3.sim <- coef(E3.sim)
 fixef.E3.sim <- fixef(E3.sim)
 ranef.E3.sim <- ranef(E3.sim)
 sigma.E3.sim <- sigma.hat(E3.sim)