Gráfico para escala likert

# É necessário instalar os pacotes a seguir. 
# install.packages("devtools")
# install.packages("plyer")
# library (devtools)
# install_github('likert', 'jbryer')
library(likert)
library(plyr)
# dados do Programme of International Student Assessment PISA
# North American (i.e. Canada, Mexico, and United States) results from the 2009 Programme of International Student Assessment (PISA) as provided by the Organization for Economic Co-operation and Development (OECD). 
# a data frame 66,690 ovservations of 81 variables from North America.
# carrega os dados na memória
data(pisaitems)
# Preparando a base de dados
# o comando substr(names(pisaitems), 1, 5) == "ST25Q"  selecionar somente as colunas com os 5 primeiros caracteres igual a ST25Q
title <- "How often do you read these materials because you want to?"
items29 <- pisaitems[, substr(names(pisaitems), 1, 5) == "ST25Q"]
names(items29) <- c("Magazines", "Comic books", "Fiction", "Non-fiction books", "Newspapers")
likert29 <- likert(items29)
summary(likert29)
plot(likert29) + ggtitle(title)

plot(likert29, centered = FALSE, plot.percents = T) + ggtitle(title)

plot(likert29, center = 2.5) + ggtitle(title)   

# por grupos de países
likert29g <- likert(items29, grouping = pisaitems$CNT)
summary(likert29g)
plot(likert29g) + ggtitle(title)

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