问题
I am trying to calculate the rolling 20 period historical volatility. I take the daily returns:
ret<-ROC(data1)
And then I use rollapply to get the 20 day HV for each column:
vol<-rollapply(ret,20,sd,by.column=T,fill=NA)
The problem is that observations in vol starts appearing after ten days which is wrong as I specified 20.
For demonstration here is sample of the data:
0.000000000, 0.005277045, 0.023622047, 0.002564103,-0.002557545, -0.020512821,
0.007853403,-0.012987013, 0.007894737, 0.015665796, 0.000000000, -0.002570694,
0.002577320, -0.015424165, 0.002610966, 0.010416667, 0.002577320, 0.015424165,
0.000000000, -0.002531646, -0.002538071, 0.030534351, 0.014814815, -0.007299270,
-0.009803922, -0.012376238, 0.002506266, -0.015000000,-0.002538071, 0.002544529
Assume the data above is stored in x, then:
rollapply(x,20,sd,fill=NA)
will yield a first observation at 10th row instead of 20. Also the sd is wrong too.
I should be missing something here...
回答1:
You need to use align='right' instead of using the default which is align='center', or instead of using rollapply, use the rollapplyr wrapper which has align='right' as the default.
From ?rollapply:
align specifyies whether the index of the result should be left- or right-aligned or centered (default) compared to the rolling window of observations. This argument is only used if width represents widths.
Although, for this, personally, I'd use runSD from the TTR package because it uses compiled code and will be faster.
Either of these should do what you expect, but the second one will be faster.
library(zoo)
rollapply(x, 20, sd, fill=NA, align='right')
library(TTR)
runSD(x, 20)
来源：https://stackoverflow.com/questions/13243148/rollapply-for-time-series