Equiprobability envelope

In Chapter 29 we introduced the concept of the two dual data spaces. Each of the n rows of the data table X can be represented as a point in the p-dimensional column-space S . In Fig. 31.2a we have represented the n rows of X by means of the row-pattern F. The curved contour represents an equiprobability envelope, e.g. a curve that encloses 99% of the points. In the case of multinormally distributed data this envelope takes the form of an ellipsoid. For convenience we have only represented two of the p dimensions of SP which is in reality a multidimensional space rather than a two-dimensional one. One must also imagine the equiprobability envelope as an ellipsoidal (hyper)surface rather than the elliptical curve in the figure. The assumption that the data are distributed in a multinormal way is seldom fulfilled in practice, and the patterns of points often possess more complex structure than is shown in our illustrations. In Fig. 31.2a the centroid or center of mass of the pattern of points appears at the origin of the space, but in the general case this needs not to be so. 

Similarly, Fig. 31.2b shows the column-pattern F of the p columns of the data table X by means of an elliptical envelope in the dual n-dimensional row-space 5". The ellipses should be interpreted as (hyper)ellipsoidal equiprobability envelopes of multinormal data. In practice the data are rarely multinormal and the centroid (or center of mass) of the pattern does not generally appear at the origin of space. An essential feature is that the equiprobability envelopes are similarly shaped in Figs. 31.2a and b. The reason for this will become apparent below. Note that in the previous section we have assumed by convention that n exceeds p, but this is not reflected in Figs. 31.2a and b. 

In Fig. 31.2a we have represented the ith row x, of the data table X as a point of the row-pattern F in column-space S . The additional axes v, and V2 correspond with the columns of V which are the column-latent vectors of X. They define the orientation of the latent vectors in column-space S. In the case of a symmetrical pattern such as in Fig. 31.2, one can interpret the latent vectors as the axes of symmetry or principal axes of the elliptic equiprobability envelopes. In the special case of multinormally distributed data, Vj and V2 appear as the major and minor... 

The same geometrical considerations can be applied to the dual representation of the column-pattern in row-space S" (Fig. 31.2b). Here u, is the major axis of symmetry of the equiprobability envelope. The projection of theyth column Xy of X upon u, is at a distance from the origin given by ... 

Fig. 6. ORTEP diagram of the charge-transfer salt showing the relative disposition of the cationic pyiidinium acceptor P(NCP+) to the anionic TpMo(CO)3 donor. The thermal ellipsoids are 20% equiprobability envelopes with hydrogens omitted for clarity (93).

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