R help - Aug 2010 - Grouping sets of data, performing function and re-assigning values

Hi there,

I hope you have time to read this question and offer a suggestion or two.

My basic question is this:  

I have data in sets of three.  I would like to combine the data from each set,
perform a function (probably just taking the median and MAD), then re-assign
these values to each of the original sets of data.

As a bit of background, I have performed a microscopy screen and analyzed the
images using software called CellProfiler.  I have three 'control'
images that I would like to combine and then compare parameters from each of my
'treatment' images to the combined control data.

However, there are a couple of 'wrinkles' to my problem: Not all the
data is actually in sets of three.   If there are no objects in a particular
field,CellProfiler does not output anything for that image.  For example, I
could have the following data set:

ImageNumber	Measurement
1				5
2				7
3				8
4				3
6				9

So in this example, I would like to combine images 1 to 3 in one set and 4 to 6
in another set (image 5 was empty).

Here is another 'wrinkle':
For each image, there are multiple measurements based on the number of objects
in the field of view I have measured.  So my data actually looks something like
this

ImageNumber	Measurement
1				4
1				5
1				5
2				6
2				7
2				8
3				8
3				9
4				1
4				1
4				6
4				3
6				10
6				9
6				5
6				8

I have attached a .csv containing a portion of the actual data (in this example,
images 1 through 3 are not present).

Right now, I can split the data.frame based on the ImageNumber column and
perform functions, but I can't come up with a way of combining the data in a
way that might account for the absence of some of the images.

Thanks for reading and any help and suggestions are much appreciated.			

JT


-------------- next part --------------

Hi all,


Since I could not attach a file to my original e-mail request, for those who
want to look at an example of a data file I am working with, please use this
link:

http://dl.dropbox.com/u/4637975/exampledata.csv

Thanks again,

Johnny.