# GSS example of chi-square test of independence
# Edps 589
# Fall 2018
# C.J.Anderson
#
#  library MASS
#          vcd
#
# Shows multiple ways to get X2 and G2 for independence
#       forshadow:  A good model for these data
# 

# Set working directory
setwd("D:\\Dropbox\\edps 589\\2 Chi-square")

# set libraries
library(MASS)
library(vcd)

#  Input data
( gss <- read.table("gss_data.txt",header=TRUE) )

( gss.tab <- xtabs(count ~ fechld + mapaid,data=gss) )

( addmargins(gss.tab) )

# Pearson chi-square statistic, df, and pvalue:                tabular data
chisq.test(gss.tab,correct=FALSE)
# or 
assocstats(gss.tab)

# Pearson and Likelihood ratio statistics, df, and pvalues:    data frame (case data)
loglm(count ~ fechld + mapaid, data=gss)

# Pearson and Likelihood ratio statistics, df, and pvalues:    tabular data
loglm( ~ fechld + mapaid, data=gss.tab)

#... and yet another way (likelihood ratio statistic is "deviance"---
#     a pre-view to latter in the semester.
# -- Need factors
fechld.f <- as.factor(gss$fechld)
mapaid.f <- as.factor(gss$mapaid)
# -- Fit model of independence
model.glm <- glm(count ~ fechld.f + mapaid.f, data=gss, family=poisson)
summary(model.glm)
# -- to get p-values
1-pchisq(44.961,12)


# A good model: "log linear x linear"....pre-view to even later in the semester
model.good <- glm(count ~ fechld.f + mapaid.f + fechld*mapaid, data=gss, family=poisson)
summary(model.good)