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Below is computer code written in the R programming language that calculates means and plots data. Just copy and paste it into R and watch it rip. The data set for this R program can be found HERE.

# First we get our data.
names(mydata) # Lets us see all the variable names.
#attach(mydata) # This puts the variable names in memory. We will not be using this.
mysubsetdata<-subset(mydata, select=c(YEAR, MTOTCONG)) #This keeps only the two variables that we need.
summary(mysubsetdata) # Since no variables are listed, a summary for all variables in the data frame is printed.
mysubsetdata #This prints out all the variable values.

# Now let us look at what the on-year/off-year pattern looks like.
# Note that to reference a variable, you need to put the data set name followed by a \$ before the variable name.
# We can reference the variables directly without using the data set name and the \$ if we used the attach command above.
# But we do not want to use the attach command now because we will be creating other data sets that contain the same variable names.
plot(mysubsetdata\$YEAR, mysubsetdata\$MTOTCONG, xlab="", ylab="", ylim=c(0.2,0.8), pch=19, type="o")
title(xlab="Year", ylab="Congressional Mobilization", main="Figure 1: Plot of U.S. Congressional Mobilization", cex=1.5, col="black", font=2)
# Now let us get the overall mean for congressional mobilization
mean(mysubsetdata\$MTOTCONG) # Here we are getting the mean only of the MTOTCONG variable.

windows() # This prevents the next plot from erasing the previous plot by giving us a new graphics window.

# Now let us work with just the years 1950 through 1970.
my5070data <- subset(mysubsetdata, YEAR >= 1950 & YEAR <= 1970)
my5070data
# attach(my5070data) # Let us avoid variable confusion with these data sets by not doing this.
plot(my5070data\$YEAR, my5070data\$MTOTCONG, xlab="", ylab="", ylim=c(0.2,0.8), pch=19, type="o", axes=FALSE)
axis(1, at=c(1952, 1956, 1960, 1964, 1968)) # This defines the X axis tick marks.
axis(2, yaxs="r") # This defines the Y axis.
box()
title(xlab="Year", ylab="Congressional Mobilization", main="Figure 2: Plot of U.S. Congressional Mobilization, 1950-70", cex=1.5, col="black", font=2)
mean(my5070data\$MTOTCONG)

windows()

# Now let us work with just the years 1972 through 1988.
my7288data <- subset(mysubsetdata, YEAR >= 1972 & YEAR <= 1988)
my7288data
# attach(my7288data) # Again, let us avoid variable confusion with these data sets by not doing this.
plot(my7288data\$YEAR, my7288data\$MTOTCONG, xlab="", ylab="", ylim=c(0.2,0.8), pch=19, type="o", axes=FALSE)
axis(1, at=c(1972, 1976, 1980, 1984, 1988)) # This defines the X axis tick marks.
axis(2, yaxs="r") # This defines the Y axis.
box()
title(xlab="Year", ylab="Congressional Mobilization", main="Figure 2: Plot of U.S. Congressional Mobilization, 1972-88", cex=1.5, col="black", font=2)
mean(my7288data\$MTOTCONG)

# Now let us work with on-year and off-year separately to get the means
myON5070data <- subset(mysubsetdata, YEAR == 1952 | YEAR == 1956 | YEAR == 1960 | YEAR == 1964 | YEAR == 1968)
mean(myON5070data\$MTOTCONG)
myOFF5070data <- subset(mysubsetdata, YEAR == 1950 | YEAR == 1954 | YEAR == 1958 | YEAR == 1962 | YEAR == 1966 | YEAR == 1970)
mean(myOFF5070data\$MTOTCONG)