The data page cluster ratio is used to compute the effectiveness of large I/O when this index is used to access the data pages. If the table is perfectly clustered with respect to the index, the cluster ratio is 1.0. Data page cluster ratios can vary widely. They are often high for some indexes, and very low for others.
See “How cluster ratios affect large I/O estimates” on page 67 in the Performance and Tuning: Optimizer for more information.
