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CPT Help Home -> Specific Menu Items -> Tools -> Modes -> Scree Plots Scree PlotsThe scree plots show the percentage of variance explained by each X and/or Y mode. The percentages are shown for all modes regardless of the numbers that were specified for use. The plots can be useful in selecting the appropriate number(s) of X and Y modes to retain. One method for selecting the number of modes involves identifying "elbows" in the scree plot. The number of modes up to, but not including, the elbow are retained on the basis that the additional modes explain similar amounts of variance, which are assumed to be largely noise. Elbows are often easier to identify when the percentage variance is plotted on a logarithmic axis. An option to use logarithmic axes is available by right-clicking on the scree plots. Another simple method for selecting the number of modes is to retain a sufficient number to ensure that the total variance retained exceeds a predefined minimum. The cumulative percentage of variance explained by the modes can be shown using the Cumulative option by right-clicking on the scree plots.Another method for selecting the number of models is to compare the explained variance of each mode to that expected from random data. A computationally and conceptually simple such approach is based on the random stick theorem, which involves randomly dividing the total variance into segments and considering the expected size of the nth largest segment. The sizes of the random stick segments can be included on the scree plot by an option available by right-clicking on the plot. The graphics can be saved individually or as a montage as a BMP, EMF, GIF, JPEG, PCX, or PNG file by right-clicking anywhere in the child window, and then selecting the desired graphic to be saved from Export. A default name is given to the graphic file, but this name can be changed using the browse button. The quality of the JPEG file can be adjusted using the slider or by the quality indicator, which ranges between 0.01 and 1.00. The highest quality is obtained using 1.00. The size of the JPEG file is affected by the quality chosen, with larger files being generated the higher the selected quality. The image quality for all the graphics formats can be improved by increasing the size of the image (see Options ~ Graphics ~ Graph Scaling). The graphic titles can be set using the Customise option upon right-clicking in the child window.
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