Compare two user defined curves and score their similarity

Question

I have a set of 2 curves (each with a few hundreds to a couple thousands datapoints) that I want to compare and get some similarity "score". Actually, I have >100 of those sets to compare... I am familiar with R (or at least bioconductor) and would like to use it.

I tried the ccf() function but I'm not too happy about it.

For example, if I compare c1 to the following curves:

c1 <- c(0, 0.8, 0.9, 0.9, 0.5, 0.1, 0.5)

c1b <- c(0, 0.8, 0.9, 0.9, 0.5, 0.1, 0.5) # perfect match! ideally score of 1

c1c <- c(1, 0.2, 0.1, 0.1, 0.5, 0.9, 0.5) # total opposite, ideally score of -1? (what would 0 be though?)

c2 <- c(0, 0.9, 0.9, 0.9, 0, 0.3, 0.3, 0.9) #pretty good, score of ???

Note that the vectors don't have the same size and it needs to be normalized, somehow... Any idea? If you look at those 2 lines, they are fairly similar and I think that in a first step, measuring the area under the 2 curves and subtracting would do. I look at the post "Shaded area under 2 curves in R" but that is not quite what I need.

A second issue (optional) is that for lines that have the same profile but different amplitude, I would like to score those as very similar even though the area under them would be big:

c1 <- c(0, 0.8, 0.9, 0.9, 0.5, 0.1, 0.5)

c4 <- c(0, 0.6, 0.7, 0.7, 0.3, 0.1, 0.3) # very good, score of ??

I hope that a biologist pretending to formulate problem to programmer is OK...

I'd be happy to provide some real life examples if needed.

Thanks in advance!

Answer 1

They don't form curves in the usual meaning of paired x.y values unless they are of equal length. The first three are of equal length and after packaging in a matrix the rcorr function in HMisc package returns:

> rcorr(as.matrix(dfrm))[[1]]
    c1 c1b c1c
c1   1   1  -1
c1b  1   1  -1
c1c -1  -1   1   # as desired if you scaled them to 0-1

The correlation of the c1 and c4 vectors:

> cor( c(0, 0.8, 0.9, 0.9, 0.5, 0.1, 0.5),
  c(0, 0.6, 0.7, 0.7, 0.3, 0.1, 0.3) )
[1] 0.9874975