Principal Component Analysis
+This calculation creates a spatial representation of the +similarities within a selected group, or all of the sequences in an +alignment. After the calculation finishes, a 3D viewer displays the set +of sequences as points in 'similarity space', and similar sequences tend +to lie near each other in the space.
+Note: The calculation is computationally expensive, and may fail +for very large sets of sequences - usually because the JVM has run out +of memory. A future release of Jalview will be able to avoid this by +executing the calculation via a web service.
+Principal components analysis is a technique for examining the +structure of complex data sets. The components are a set of dimensions +formed from the measured values in the data set, and the principle +component is the one with the greatest magnitude, or length. The sets of +measurements that differ the most should lie at either end of this +principle axis, and the other axes correspond to less extreme patterns +of variation in the data set.
+ +In this case, the components are generated by an eigenvector +decomposition of the matrix formed from the sum of BLOSUM scores at each +aligned position between each pair of sequences. The matrix is not +symmetric - elements in the upper diagonal give the sum of scores for +mutating in one direction, and the lower diagonal is the sum of scores +for mutating in the other. This is a refinement of the method described +in the paper by G. Casari, C. Sander and A. Valencia. Structural Biology +volume 2, no. 2, February 1995 (pubmed) +and implemented at the SeqSpace server at the EBI.
+ +The PCA Viewer
+This is an interactive display of the sequences positioned within +the similarity space, as points in a rotateable 3D scatterplot. The +colour of each sequence point is the same as the sequence group colours, +white if no colour has been defined for the sequence, and green if the +sequence is part of a the currently selected group.
+The 3d view can be rotated by dragging the mouse with the left +mouse button pressed. The view can also be zoomed in and out with the up +and down arrow keys (and the roll bar of the mouse if +present). Labels will be shown for each sequence if the entry in the +View menu is checked, and the plot background colour changed from the +View→Background Colour.. dialog box. The File menu allows the view +to be saved (File→Save submenu) as an EPS or PNG +image or printed, and the original alignment data and matrix resulting +from its PCA analysis to be retrieved. The coordinates for the whole PCA +space, or just the current view may also be exported as CSV files for +visualization in another program or further analysis.
+A tool tip gives the sequence ID corresponding to a point in the +space, and clicking a point toggles the selection of the corresponding +sequence in the associated alignment window views. Rectangular region +based selection is also possible, by holding the 'S' key whilst +left-clicking and dragging the mouse over the display. By default, +points are only associated with the alignment view from which the PCA +was calculated, but this may be changed via the View→Associate +Nodes sub-menu.
+Initially, the display shows the first three components of the +similarity space, but any eigenvector can be used by changing the +selected dimension for the x, y, or z axis through each ones menu +located below the 3d display.
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