Supermarket Prices Narrative a Building Contractor Employs Three Construction Engineers, A, B, and A

Supermarket Prices Narrative
A building contractor employs three construction engineers, A, B, and C to estimate and bid on jobs. To determine whether one tends to be a more conservative (or liberal) estimator than the others, the contractor selects four projected construction jobs and has each estimator independently estimate the cost (in dollars per square foot) of each job. The data are shown in the table:
Construction Job
-Refer to Supermarket Prices Narrative. Why is it important to treat the food items as blocks? (Hint: The observed value of the test statistic in a completely randomized design is F = 0.089, and = 2.69).