Five supermarket checkers were randomly selected to test two different checkout procedures. The order in which the procedures were assigned to each checker was random. The number of items checked in a half hour period was recorded for each checker using each procedure. Results are shown in the table below. Assuming that all necessary conditions are satisfied, build a 99% confidence interval estimate of the difference in average number of items checked for the populations represented.

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Items Checked Checker Procedure 1 654 721 597 612 689 Procedure 2 603 658 615 590 642