why change days of the week from a factor to an ordered factor?

f1 <- factor(1:5, lev = 5:1)
f2 <- factor(1:5,lev=5:1,ordered=TRUE)
identical(f1,f2)

## They are different classes!!
f1

f2

sort(f1)

sort(f2)

On Dec 2, 2013, at 6:58 PM, Bill wrote:

Duncan,
Thanks. Why doesn't
coloursf2 <- factor(1:8, levels = 8:1)

give an ordering when you do str(coloursf2) like
"8"<"7"<"6" ...

Because the default for 'ordered' in factor is FALSE:

 coloursf2 <- factor(1:8, levels = 8:1, ordered=TRUE)
coloursf2

[1] 1 2 3 4 5 6 7 8
Levels: 8 < 7 < 6 < 5 < 4 < 3 < 2 < 1

Bill


On Mon, Dec 2, 2013 at 6:29 PM, Duncan Mackay <dulcalma at bigpond.com> wrote:

Hi Bill

eg

colours =  1:8
coloursf =  factor(1:8)
colourso =  ordered(1:8)
str(coloursf)

Factor w/ 8 levels "1","2","3","4",..: 1 2 3 4 5 6 7 8

str(colourso)

Ord.factor w/ 8 levels "1"<"2"<"3"<"4"<..: 1 2 3 4 5 6 7 8

coloursf2 <- factor(1:8, levels = 8:1)
str(coloursf2)

Duncan

Duncan
Duncan Mackay
Department of Agronomy and Soil Science
University of New England
Armidale NSW 2351
Email: home: mackay at northnet.com.au


ordered used in
used in MASS::polr and GEE for polytomous logistic regression

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On
Behalf Of Bill
Sent: Monday, 2 December 2013 21:24
To: r-help at r-project.org
Subject: [R] why change days of the week from a factor to an ordered
factor?

I am reading the code below. It acts on a csv file called dodgers.csv with
the following variables.

print(str(dodgers))  # check the structure of the data frame

'data.frame':   81 obs. of  12 variables:
$ month      : Factor w/ 7 levels "APR","AUG","JUL",..: 1 1 1 1 1 1 1 1 1
1 ...
$ day        : int  10 11 12 13 14 15 23 24 25 27 ...
$ attend     : int  56000 29729 28328 31601 46549 38359 26376 44014 26345
44807 ...
$ day_of_week: Factor w/ 7 levels "Friday","Monday",..: 6 7 5 1 3 4 2 6 7
1 ...
$ opponent   : Factor w/ 17 levels "Angels","Astros",..: 13 13 13 11 11 11
3 3 3 10 ...
$ temp       : int  67 58 57 54 57 65 60 63 64 66 ...
$ skies      : Factor w/ 2 levels "Clear ","Cloudy": 1 2 2 2 2 1 2 2 2 1
...
$ day_night  : Factor w/ 2 levels "Day","Night": 1 2 2 2 2 1 2 2 2 2 ...
$ cap        : Factor w/ 2 levels "NO","YES": 1 1 1 1 1 1 1 1 1 1 ...
$ shirt      : Factor w/ 2 levels "NO","YES": 1 1 1 1 1 1 1 1 1 1 ...
$ fireworks  : Factor w/ 2 levels "NO","YES": 1 1 1 2 1 1 1 1 1 2 ...
$ bobblehead : Factor w/ 2 levels "NO","YES": 1 1 1 1 1 1 1 1 1 1 ...
NULL

I don't understand why the author of the code decided to make the factor
days_of_week into an ordered factor. Anyone know why this should be done?
Thank you.

Here is the code:

# Predictive Model for Los Angeles Dodgers Promotion and Attendance

library(car)  # special functions for linear regression
library(lattice)  # graphics package

# read in data and create a data frame called dodgers dodgers <-
read.csv("dodgers.csv")
print(str(dodgers))  # check the structure of the data frame

# define an ordered day-of-week variable # for plots and data summaries
dodgers$ordered_day_of_week <- with(data=dodgers,
 ifelse ((day_of_week == "Monday"),1,
 ifelse ((day_of_week == "Tuesday"),2,
 ifelse ((day_of_week == "Wednesday"),3,
 ifelse ((day_of_week == "Thursday"),4,
 ifelse ((day_of_week == "Friday"),5,
 ifelse ((day_of_week == "Saturday"),6,7)))))))
dodgers$ordered_day_of_week
<- factor(dodgers$ordered_day_of_week,
levels=1:7,
labels=c("Mon", "Tue", "Wed", "Thur", "Fri", "Sat", "Sun"))

# exploratory data analysis with standard graphics: attendance by day of
week with(data=dodgers,plot(ordered_day_of_week, attend/1000, xlab = "Day
of
Week", ylab = "Attendance (thousands)", col = "violet", las = 1))

# when do the Dodgers use bobblehead promotions with(dodgers,
table(bobblehead,ordered_day_of_week)) # bobbleheads on Tuesday

# define an ordered month variable
# for plots and data summaries
dodgers$ordered_month <- with(data=dodgers,
 ifelse ((month == "APR"),4,
 ifelse ((month == "MAY"),5,
 ifelse ((month == "JUN"),6,
 ifelse ((month == "JUL"),7,
 ifelse ((month == "AUG"),8,
 ifelse ((month == "SEP"),9,10)))))))
dodgers$ordered_month <- factor(dodgers$ordered_month, levels=4:10, labels
=
c("April", "May", "June", "July", "Aug", "Sept", "Oct"))

# exploratory data analysis with standard R graphics: attendance by month
with(data=dodgers,plot(ordered_month,attend/1000, xlab = "Month", ylab =
"Attendance (thousands)", col = "light blue", las = 1))

# exploratory data analysis displaying many variables # looking at
attendance and conditioning on day/night # the skies and whether or not
fireworks are displayed
library(lattice) # used for plotting
# let us prepare a graphical summary of the dodgers data group.labels <-
c("No Fireworks","Fireworks") group.symbols <- c(21,24) group.colors <-
c("black","black") group.fill <- c("black","red")
xyplot(attend/1000 ~ temp | skies + day_night,
   data = dodgers, groups = fireworks, pch = group.symbols,
   aspect = 1, cex = 1.5, col = group.colors, fill = group.fill,
   layout = c(2, 2), type = c("p","g"),
   strip=strip.custom(strip.levels=TRUE,strip.names=FALSE, style=1),
   xlab = "Temperature (Degrees Fahrenheit)",
   ylab = "Attendance (thousands)",
   key = list(space = "top",
       text = list(rev(group.labels),col = rev(group.colors)),
       points = list(pch = rev(group.symbols), col = rev(group.colors),
       fill = rev(group.fill))))

# attendance by opponent and day/night game group.labels <-
c("Day","Night")
group.symbols <- c(1,20) group.symbols.size <- c(2,2.75) bwplot(opponent ~
attend/1000, data = dodgers, groups = day_night,
   xlab = "Attendance (thousands)",
   panel = function(x, y, groups, subscripts, ...)
      {panel.grid(h = (length(levels(dodgers$opponent)) - 1), v = -1)
       panel.stripplot(x, y, groups = groups, subscripts = subscripts,
       cex = group.symbols.size, pch = group.symbols, col = "darkblue")
      },
   key = list(space = "top",
   text = list(group.labels,col = "black"),
   points = list(pch = group.symbols, cex = group.symbols.size,
   col = "darkblue")))

# specify a simple model with bobblehead entered last my.model <- {attend ~
ordered_month + ordered_day_of_week + bobblehead}

# employ a training-and-test regimen
set.seed(1234) # set seed for repeatability of training-and-test split
training_test <- c(rep(1,length=trunc((2/3)*nrow(dodgers))),
rep(2,length=(nrow(dodgers) - trunc((2/3)*nrow(dodgers)))))
dodgers$training_test <- sample(training_test) # random permutation
dodgers$training_test <- factor(dodgers$training_test,
 levels=c(1,2), labels=c("TRAIN","TEST")) dodgers.train <- subset(dodgers,
training_test == "TRAIN")
print(str(dodgers.train)) # check training data frame dodgers.test <-
subset(dodgers, training_test == "TEST")
print(str(dodgers.test)) # check test data frame

# fit the model to the training set
train.model.fit <- lm(my.model, data = dodgers.train) # obtain predictions
from the training set dodgers.train$predict_attend <-
predict(train.model.fit)

# evaluate the fitted model on the test set dodgers.test$predict_attend <-
predict(train.model.fit,
 newdata = dodgers.test)

# compute the proportion of response variance # accounted for when
predicting out-of-sample cat("\n","Proportion of Test Set Variance
Accounted
for: ", round((with(dodgers.test,cor(attend,predict_attend)^2)),
 digits=3),"\n",sep="")

# merge the training and test sets for plotting dodgers.plotting.frame <-
rbind(dodgers.train,dodgers.test)

# generate predictive modeling visual for management group.labels <- c("No
Bobbleheads","Bobbleheads") group.symbols <- c(21,24) group.colors <-
c("black","black") group.fill <- c("black","red")
xyplot(predict_attend/1000 ~ attend/1000 | training_test,
      data = dodgers.plotting.frame, groups = bobblehead, cex = 2,
      pch = group.symbols, col = group.colors, fill = group.fill,
      layout = c(2, 1), xlim = c(20,65), ylim = c(20,65),
      aspect=1, type = c("p","g"),
      panel=function(x,y, ...)
           {panel.xyplot(x,y,...)
            panel.segments(25,25,60,60,col="black",cex=2)
           },
      strip=function(...) strip.default(..., style=1),
      xlab = "Actual Attendance (thousands)",
      ylab = "Predicted Attendance (thousands)",
      key = list(space = "top",
             text = list(rev(group.labels),col = rev(group.colors)),
             points = list(pch = rev(group.symbols),
             col = rev(group.colors),
             fill = rev(group.fill))))

# use the full data set to obtain an estimate of the increase in #
attendance due to bobbleheads, controlling for other factors my.model.fit
<-
lm(my.model, data = dodgers)  # use all available data
print(summary(my.model.fit))
# tests statistical significance of the bobblehead promotion # type I anova
computes sums of squares for sequential tests
print(anova(my.model.fit))

cat("\n","Estimated Effect of Bobblehead Promotion on Attendance: ",
round(my.model.fit$coefficients[length(my.model.fit$coefficients)],
digits = 0),"\n",sep="")

# standard graphics provide diagnostic plots
plot(my.model.fit)

# additional model diagnostics drawn from the car package
library(car)
residualPlots(my.model.fit)
marginalModelPlots(my.model.fit)
print(outlierTest(my.model.fit))

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David Winsemius
Alameda, CA, USA