Two tutoring services offer crash courses in preparation for the CPA exam. To check on the effectiveness of these services, twenty-one students were chosen. Seven students were randomly assigned to service A, seven were assigned to service B, and the remaining seven did not take a crash course. Their scores on the examination, expressed as percentages, are given in the following table.

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Service A 61 65 71 52 67 58 78 Service B 91 88 74 56 76 81 66 No Course 99 83 75 95 62 69 80