Calculate percent change from a baseline year (t0) to a subsequent BUT LIMITED series of years (t1, ..., tk)

Question

Imagine you have yearly data for some sort of expenses. You are interested in the percent difference between the first value (t0) and each subsequent value (t1, ... -> tx) BUT only for a specific group of observations, i.e. with the next group, a new series of subsequent years starts.

Example:

    value <- c(10225,10287,10225,10087,10344,10387,10387,14567,13992,15432)
    case <- c(A,A,A,B,B,B,B,B,C,C)

    year    value   case   change
    1989    10225   A      0.00
    1990    10287   A      0.61 # ((100/10225)*10287)-100
    1991    10262   A      0.36
    1995    10087   B      0.00
    1996    10344   B      2.55 # ((100/10087)*10344)-100
    1997    10387   B      2.97 
    1978    10387   B      2.97
    1979    14567   B      ...
    1980    13992   C
    1981    15432   C

How can I calculate the percent change in R?

我之前的帖子和类似帖子（例如，这篇关于计算相对差异的帖子）的答案 非常有帮助。再次感谢！

但是，我不得不意识到我的案情更加复杂，因此对问题进行了编辑。问题是我没有一系列的后续年份，但有一系列有限的后续年份，每组病例一个。

任何想法都受到高度赞赏！

非常感谢。

Answer 1

那这个呢？

((value[-1]/value[1])-1)*100
[1]  0.6063570  0.0000000 -1.3496333  1.1638142  1.5843521  0.7334963

另一种选择

((value - value[1]) / value[1]) * 100
[1]  0.0000000  0.6063570  0.0000000 -1.3496333  1.1638142  1.5843521  0.7334963

对于您的更新问题，这是两个R base解决方案：

transform(df, Change = unlist(sapply(split(value, case), function(x) ((x - x[1]) / x[1]) * 100)))
   value case    Change
A1 10225    A  0.000000
A2 10287    A  0.606357
A3 10225    A  0.000000
B1 10087    B  0.000000
B2 10344    B  2.547834
B3 10387    B  2.974125
B4 10387    B  2.974125
B5 14567    B 44.413602
C1 13992    C  0.000000
C2 15432    C 10.291595

 transform(df, Change = unlist(aggregate(value ~ case, function(x) ((x - x[1]) / x[1]) * 100, data=df)$value))
   value case    Change
01 10225    A  0.000000
02 10287    A  0.606357
03 10225    A  0.000000
11 10087    B  0.000000
12 10344    B  2.547834
13 10387    B  2.974125
14 10387    B  2.974125
15 14567    B 44.413602
21 13992    C  0.000000
22 15432    C 10.291595