for this blog post: https://politbistro.hypotheses.org/3146 – for background, see https://ingorohlfing.wordpress.com/2016/02/24/the-rise-of-process-tracing-in-google-books/

pt-ngrams.r

# library(devtools)
# install_github("ngramr", "seancarmody")
library(ngramr)
library(ggplot2)

pt <- ngram("process tracing", year_start = 1970)
ql <- ngram("qualitative methods", year_start = 1970)
qt <- ngram("quantitative methods", year_start = 1970)
rm <- ngram("research methods", year_start = 1970)
cs <- ngram("case study methods", year_start = 1970)
cr <- ngram("case study research", year_start = 1970)

ggplot(pt, aes(x = Year, y = Frequency, colour = Phrase)) +
  geom_vline(xintercept = c(1991, 2005), lty = "dotted") +
  geom_line()

ggsave("pt-1.png", width = 7.5, height = 5)

all <- ngram(c("process tracing",
               "qualitative methods",
               "quantitative methods",
               "research methods",
               "case study methods",
               "case study research"),
             year_start = 1970)

ggplot(all, aes(x = Year, y = Frequency, colour = Phrase)) +
  geom_vline(xintercept = c(1991, 2005), lty = "dotted") +
  geom_line()

ggsave("pt-2.png", width = 7.5, height = 5)

ggplot(pt, aes(x = Year, y = Frequency, colour = Phrase)) +
  geom_vline(xintercept = c(1991, 2005), lty = "dotted") +
  geom_line(aes(y = Frequency / ql$Frequency, color = "pt / qual")) +
  geom_line(aes(y = Frequency / qt$Frequency, color = "pt / quant")) +
  geom_line(aes(y = Frequency / (ql$Frequency + qt$Frequency), color = "pt / qual + quant")) +
  geom_line(aes(y = Frequency / rm$Frequency, color = "pt / res. methods"))

ggsave("pt-3.png", width = 7.5, height = 5)

ggplot(pt, aes(x = Year, y = Frequency, colour = Phrase)) +
  geom_vline(xintercept = c(1991, 2005), lty = "dotted") +
  geom_line(aes(y = Frequency / cs$Frequency, color = "pt / case st methods")) +
  geom_line(aes(y = Frequency / cr$Frequency, color = "pt / case st research")) +
  geom_line(aes(y = Frequency / (cs$Frequency + cr$Frequency), color = "pt / case st (r + m)"))

ggsave("pt-4.png", width = 7.5, height = 5)