In the previous post, I showed code and some output related to our topic for this post. You'll need to generate those numbers to follow along. Of course, you could just paste 'em in:

The whole trick to PCoA is that it can take a "measure of association" between two samples, like a distance. So, let's use the values from before, calculate the distances between them, and then see how well PCoA recovers the original data. The second half of the code listing is at the bottom of the post. In the first segment of this second half, we simply define two functions to calculate the distances between all the points, using either the "manhattan" distance or the "euclidean" distance. We will only use the euclidean version for this analysis. If you want to see the results of the distance calculation, you'll have to run the code (manhattan) or modify it to print the euclidean result.

Next, we want to do the PCoA analysis. I did this two ways (to check myself). First, I looked under the hood at the PyCogent module

`cogent.cluster.metric_scaling`

, and I call the same functions used in ` metric_scaling.principal_coordinates_analysis`

. These include the following two functions as well as a `numpy.linalg`

routine:followed by a little massaging of the vectors to generate the output. As you will see if you run the code, the eigenvalues and then the last two rows of my result are:

The other approach is to call PyCogent's function directly. The eigenvalues and the last two results of the call to

are

This looks pretty good, though I notice that my eigenvalues aren't matching. I'm missing a square root somewhere. I will postpone worrying about this for another day.

The original matrix is:

I think this is pretty remarkable. We've taken a matrix of distances between our sample points, done a little PCoA mumbo-jumbo, and regenerated something pretty close to the original.

However, I am a bit worried at what look to be bugs in the implementation:

• the original x-values are systematically transformed by + 2

• the original y-values are systematically transformed by * -1

This gives a shift and a reflection of the points, but the relationships revealed by the PCoA are identical to the original.

[UPDATE: <whacks self on head> Duh...I bet that in view of the last line above, if we calculate the distance we'll get the same result as we did for the original matrix. So, of course, there's no reason that PCoA could possibly tell the difference. Presumably, it's just centered its result a little better around (0,0). ]