Code to copy and paste for class 8.

ps230classwork8.R

##Load a small version of Fortna's data.
load(url("http://jakebowers.org/PS230Data/pk2.df.rda")) 
## Make a new dataframe with no missing values and adding UNpk (UN
## peacekeeping) and year the civil war started 
pk.good<-na.omit(subset(pk.df,select=c('ridp','wardur','warstart.yr','warstart.hist.yr','UNpk','id')))
## Load the mosaic library for plotting etc
library(mosaic)

##Recall that 
lm3<-lm(ridp~UNpk,data=pk.good)
signif(coef(lm3),4)

lm.big<-lm(ridp~UNpk+warstart.yr+UNpk:warstart.yr,data=pk.good)
coef(lm.big)
lm.small<-lm(ridp~warstart.yr,data=pk.good)
coef(lm.small)

## Min and max of the warstart.yr variable
range(pk.good$warstart.yr)
## range(pk.good$UNpk)

preddat<-expand.grid(UNpk=c(0,1),
  				 warstart.yr=c(0,53))
preddat
preddat$lm.big.fit<-predict(lm.big,newdata=preddat)
preddat

table(1945+pk.good$warstart.yr[pk.good$UNpk==1])
table(pk.good$warstart.yr[pk.good$UNpk==1])

lm.fair<-lm(ridp~UNpk*warstart.yr,data=pk.good,subset=(1945+warstart.yr)>=1963)
lm.fair

preddat2<-expand.grid(UNpk=c(0,1),warstart.yr=c(1963-1945,1998-1945)) 
preddat2$fit<-predict(lm.fair,newdata=preddat2)
preddat2

summary(residuals(lm.fair))