egypt=read.csv("S:/Courses/stat-renaes/Stat404/Egypt skulls.csv",header=T)
head(egypt)
attach(egypt)

# boxplot
boxplot(breadth~era)

# histograms by era

# you will need to install a package called lattice

install.packages("lattice")
library(lattice)

histogram(~breadth|era, type='count', main="Egyptian Skull Breadths \n BCE200 and BCE4000")

# Now for some basic summary statistics.  There is a package that can give you all of the 
# needed statistics (mean, median, standard deviation, variance, sample size, etc.)
# when you have a grouping variable (the era). If you just had different quantitative variables with 
# no grouping variable, then you'd use a different one called stat.desc in the pastecs package

install.packages("psych")
library(psych)

describeBy(breadth,era)

# You will need the tilde (~) here to denote the grouping variable since there are not two
# separate quantitative variables (if you did have two separate variables,
# then you would just separate them by a comma)

# variance test first
var.test(breadth~era)

# now t-test
t.test(breadth~era,var.equal=T)

# can do one tail if needed:
t.test(breadth~era,var.equal=T,alternative="greater")

detach(egypt)