从纵向数据中选择最后一个观察

Question

我有一个数据集,每个参与者都有几个时间评估.我想为每个参与者选择最后一次评估.我的数据集如下所示:

ID  week  outcome
1   2   14
1   4   28
1   6   42
4   2   14
4   6   46
4   9   64
4   9   71
4  12   85
9   2   14
9   4   28
9   6   51
9   9   66
9  12   84

我想只为每个参与者选择最后一次观察/评估,但我只有周数作为每个参与者的指标.这怎么可能在R(或excel？)

提前致谢,

尼基

Answer 1

这是一个基础R方法:

do.call("rbind", 
        by(df, INDICES=df$ID, FUN=function(DF) DF[which.max(DF$week), ]))
  ID week outcome
1  1    6      42
4  4   12      85
9  9   12      84

或者,该data.table软件包提供了一种简洁而富有表现力的语言,用于执行此类数据框操作:

library(data.table)
dt <- data.table(df, key="ID")

dt[, .SD[which.max(outcome), ], by=ID] 
#      ID week outcome
# [1,]  1    6      42
# [2,]  4   12      85
# [3,]  9   12      84

# Same but much faster. 
# (Actually, only the same as long as there are no ties for max(outcome)..)
dt[ dt[,outcome==max(outcome),by=ID][[2]] ]   # same, but much faster.

# If there are ties for max(outcome), the following will still produce
# the same results as the method using .SD, but will be faster
i1 <- dt[,which.max(outcome), by=ID][[2]]
i2 <- dt[,.N, by=ID][[2]]
dt[i1 + cumsum(i2) - i2,]

最后,这是一个plyr基于解决方案

library(plyr)

ddply(df, .(ID), function(X) X[which.max(X$week), ])
#   ID week outcome
# 1  1    6      42
# 2  4   12      85
# 3  9   12      84

Answer 2

如果您只是在寻找每个人ID的最后一个观察点,那么应该使用简单的两行代码.在可能的情况下,我总是寻求简单的基础解决方案,而有多种方法来解决问题总是很好的.

dat[order(dat$ID,dat$Week),]  # Sort by ID and week
dat[!duplicated(dat$ID, fromLast=T),] # Keep last observation per ID

   ID Week Outcome
3   1    6      42
8   4   12      85
13  9   12      84