两个连续事件之间的天数

Question

我正在尝试生成变量diff来表示两个连续事件之间的天数。

library(outbreaks)
df <- measles_hagelloch_1861[order(measles_hagelloch_1861$date_of_prodrome), ] 
library(lubridate)
library(plyr)

# First date
firs_date <- min(df$date_of_prodrome)

# Commulative number of days
df$cum_number_day <- difftime(df$date_of_prodrome, firs_date, units = 'days')
head(df$cum_number_day)

# The number of days between two consecutive events
df$diff <- difftime(lag(df$date_of_prodrome, 1), df$date_of_prodrome, units = "days" )
head(df$diff)

和结果

Time differences in days
[1]  0  2  8  8  9 12
Time differences in days
[1] 0 0 0 0 0 0

您能解释一下为什么我在第一个命令中得到了期望的结果，而在第二个命令中却得到了0 0 0吗？

Answer 1

问题是您的代码使用stats::lag而不是dplyr::lag。看到不同之处：

df$diff <- difftime(stats::lag(df$date_of_prodrome, 1),df$date_of_prodrome, units = "days" )
head(df$diff)

#Time differences in days
#[1] 0 0 0 0 0 0

df$diff <- difftime(dplyr::lag(df$date_of_prodrome, 1), df$date_of_prodrome, units = "days" )
head(df$diff)

#Time differences in days
#[1] NA -2 -6  0 -1 -3