[R] Sum every n (4) observations by group

[Rasmus Liland]

Dear Miluji,

I went through different combinations, 
and find it elegant to sum the values 
with respect to ID + High/Low Date 
levels like if this was a list of 
Dose/Material/Group experiments:

	x[x$Date %in% c(4143, 4147),"j"] <-
	tapply(x$Value, paste0(x$ID, 
		ifelse(x$Date>4143, 2, 1)), sum)

This way, you escape the «every 4th 
observation» criterium, IMHO instead 
«using» the dataframe in contrast to 
creating new information or interpreting 
the structure of the data frame ...  

I'm sorry if this only creates confusion 
and a «what is he talking about?!» 
awkwardness by not answering the 
question in a precise way ... 

Best,
Rasmus