# edps 589
# c.j.anderson
#
# Symmetry for Siskell & Ebert data set:
#   Did Siskell and Ebert agree with each other?
#   Source: Chance.  
#
library(vcd)
library(vcdExtra)

def <- expand.grid(Ebert<-c("Con","Mixed","Pro"),
                  Siskel<-c("Con","Mixed","Pro"))
movies <- data.frame(def,count=c(24,8,13,8,13,11,10,9,64))
names(movies)<- c("Ebert","Siskel","count")
movies.tab <- xtabs(count ~ Siskel + Ebert,data=movies)
(addmargins(movies.tab))

#
# Independence
#
summary(ind <- glm(count~ Siskel + Ebert,data=movies,family=poisson))

#
# Quasi Independence
#
movies$qi <- c(1,0,0,
               0,2,0,
			   0,0,3)
movies$qi <- as.factor(movies$qi)
summary(qi.fit <- glm(count~Siskel+Ebert+qi, data=movies,family=poisson))
round(1-pchisq(qi.fit$deviance,qi.fit$df.residual),digits=2)

#
# Symmetry
#
movies$sym <- c(1,4,5,
               4, 2,6,
			   5, 6,3)
movies$sym <- as.factor(movies$sym)
summary(sym.fit <- glm(count~sym, data=movies,family=poisson))
round(1-pchisq(sym.fit$deviance,sym.fit$df.residual),digits=2)