What conditions would justify the assumption of a constant contribution margin per customer? Do you think those conditions are likely to hold here?

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Attachment 1 Purchases and Purchase Cost for Randomly Selected Customers Julie randomly selected 60 customer invoices 30 from day customers, 30 from evening customers. The purchase revenue and cost data are shown below. (For example, the first day customer made a purchase of $32.50, and the alcohol purchased cost $23.89 to stock.) DAY Purchase Revenue EVENING Purchase Revenue Cost Cost 32.50 23.89 40.29 32.00 35.22 25.90 34.66 25.20 39.99 30.40 35.87 24.01 23.08 16.53 36.40 28.60 39.48 28.32 30.95 24.95 30.55 20.17 33.69 22.42 24.77 18.49 33.82 25.89 30.13 21.11 34.92 23.07 31.16 24.91 35.15 25.39 34.67 25.75 32.89 22.99 32.37 25.03 36.30 28.63 40.71 29.61 38.44 29.51 33.07 23.31 32.49 25.36 43.86 29.51 34.91 25.26 45.88 36.39 52.78 37.08 42.31 29.83 37.43 28.81 36.69 29.89 30.20 19.86 43.59 28.51 37.99 26.54 31.38 23.55 26.90 19.19 32.09 21.66 47.19 34.88 26.78 17.93 33.48 25.12 26.39 33.84 18.38 44.20 36.09 22.96 33.52 22.76 32.39 25.24 37.15 26.24 25.29 17.72 35.81 26.79 21.11 14.41 37.61 25.68 27.60 22.23 30.29 24.51 37.79 25.50 31.07 23.27 38.37 29.00 24.35 19.50 27.14 21.31 34.90 28.07