Hardness-Intensity Diagrams (HIDs), where HR$_2$ is plotted versus the total count rate. Although these diagrams contain absolute flux information and depend strongly on the long term variability of the source, in some case they are useful for characterizing the behavior during a single observation.

DATA ANALYSIS
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Selection of observations (classes)
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Despite the extraordinarily complex variability displayed by GRS 1915+105, the examination of light curves and CDs shows that many features repeat in different observations. There are cases of observation intervals more than one year apart which are virtually indistinguishable from each other. We found that it was possible to classify our 349 observation intervals into only 12 classes, based on the appearance of light curves and CDs (see Table 1). This empirical classification is intended as a first step to reduce the complexity of the amount of available data, as well as a way to present the overall picture of timing/spectral variability of GRS 1915+105 in a model-independent fashion. We outline the 12 classes below. One example for each class (light curve and CD) is shown in Figure 2.

$\bullet$ class $\phi$

:   Two types of observations are relatively straightforward to separate from the others: those displaying no 