CHAPTER 04 VISUAL LAB - Check the denominator by hand
Book boards: 04a and 04b

PRACTICE ROUTINE
Cover the board answer. Predict aloud, try the example, compare with the
stated result, change one input, and try again. Explain a discrepancy
before changing another thing. All marketing examples are fictional.

Use boards 04a and 04b. Use paper and a calculator. The starter data are also
in companion/start_here/campaigns.csv, but no Python is required here.

REPRODUCE 04a
Add costs 300, 120 and 200: total EUR 620. Add enquiry counts 12, 8 and 10:
total 30. Divide the totals: 20.666... euros per enquiry, shown as EUR 20.67.
Separately average rates 25, 15 and 20: the result is 20. Explain why this
second calculation gives every campaign equal weight rather than preserving
the differing numbers of enquiries.

REPRODUCE 04b
For 8% becoming 10%, subtract the rates to get 2 percentage points. Divide
that difference by the original 8%: 2/8 is 0.25, or a 25% relative increase.

CHANGE, PREDICT, CHECK
Add a fourth campaign: EUR 180 and six enquiries. Predict total cost, total
enquiries and overall cost per enquiry before using the calculator.
Answer: EUR 800, 36 enquiries and approximately EUR 22.22 per enquiry.
If the fourth count is missing rather than six, the complete rate is unknown;
do not substitute zero simply to complete the division.

SECOND CHECK
5% to 6% is one percentage point, or a 20% relative increase. The calculations
describe observed rates and do not prove that a particular change caused them.
