Procrustes analysis

1. Procrustes analysis • Purpose of procrustes analysis • Algorithm • Various modifications

2. Purpose of procrustes analysis There are many situations when slightly different techniques produce different configurations. For example when metric and non-metric scaling are used then different configurations may be generated. Even if metric scaling is used different proximity (dissimilarity) matrices can produce different configurations. Since this techniques are used for the same multivariate observations each observation in one configuration corresponds to exactly one observation in another configuration. Most of the techniques produce configuration that rotationally is not defined. Scores in factor analysis can also be considered as one of the possible configurations. There are other situations when comparison of configurations is needed. For example in macromolecular biology 3-dimensional structures of different proteins are derived. One of the interesting question is if two different proteins are similar if they are what is similarity between them. To find similarity it is necessary to match configurations of two protein structures. All these questions can be addressed using procrustes analysis. Suppose we have two configurations X=(x1,x2,,,xn) and Y = (y1,y2,,,yn). where each x and y are vectors in p dimensional space. We want to find a orthogonal matrix A and b vector b so that:

3. Prcucrustes analysis: vector and matrix It can be show that finding translation (b) and rotation matrix (A) can be considered separately. Translation can easily be found if we centre each configuration. If rotation is already known then we can find translation. Let us denote zi=Ayi+b. Then we can write: It is minimised when centres of x and z coincide. I.e. We want centroids of the configuration to match. It can be done if we will subtract from x and y their centroids respectively. Remaining problems is finding the orthogonal matrix (matrix of rotation or inverse). We can write: Here we used the fact that under trace matrix can commute and A is the orthogonal matrix: Then we want to want to perform constrained maximisation: We can do using Lagrange’s multipliers technique.

4. Rotation matrix using SVD Let us define symmetric matrix of constraints by 1/2. Then we want to maximise: If we get derivatives of this expression wrt to matrix A and equate them to 0 then we can get: Here we used the following facts: and remembering that matrix of constraints is symmetric. We have necessary linear equations to find the required orthogonal matrix. Let us use SVD of YTX: V and U are pxp orthogonal matrices. D is the diagonal matrix of the singular values.

5. Rotation matrix and SVD If we use the fact that A is orthogonal then we can write: and It gives the solution for the rotation (orthogonal) matrix. Now we can calculate least-squares differences between configurations: Thus we have expressions for rotation matrix and differences between configurations after matching. It is interesting to note that to find differences between configurations it is not necessary rotate one of them. This expression can also be written: One more useful expression is: This expression shows that it is even not necessary to do SVD to find differences between configurations. (For square root of matrix Cholesky decomposition could be used)

6. Some modifications There are some situations where problems can occur: • Dimensions of configurations can be different. There are two ways of handling this problem. First way is to fill low dimensional (k) space with 0-s and make it high (p) dimensional. This way we assume that first k dimensions coincide. Here we assume that k-dimensional configuration is in the k-dimensional subspace of p-dimensional space. Second way is to collapse high dimensional configuration to low dimensional space. For this we need to project p-dimensional configuration to k-dimensional space. • Second problem is when the scales of the configurations are different. In this case we can add scale factor to the function we want to minimise: If we find orthogonal matrix as before then we can find expression for the scale factor: As a result M is no longer symmetric wrt X and Y. 3) Sometime it is necessary to weight some variables down and other up. In this case procrustes analysis can be performed using weights. We want to minimise the function: This modification can be taken into account. Analysis becomes easy when weight matrix is diagonal.