01
Define the entries and the missing-value rule
Create columns for collection, card identifier, width in millimetres and observation status. Enter A3 as unknown rather than as zero. The inclusion rule for this worksheet is to use only supplied numeric widths. A therefore contributes two measured entries out of three listed cards, while B contributes three out of three. Keep the listed count and the measured count visible together. A future observation of A3 would add information to the dataset; it would not correct a measured zero because no zero was supplied in the first place.
02
Calculate means using the measured counts
For A, add 40 and 50 to obtain 90 mm, then divide by two measured cards to obtain a mean width of 45 mm. For B, add 42, 48 and 60 to obtain 150 mm, then divide by three to obtain 50 mm. The observed means differ by 5 mm. Dividing A’s sum by all three listed cards would produce 30 mm and silently treat the missing width as zero. Write the sum and denominator beside each mean so that another reader can inspect the calculation directly.
03
Keep the conclusion attached to the observed set
The reusable worksheet records the quantity, unit, inclusion rule, listed count, measured count, sum, mean and unresolved entries. Its conclusion is limited to these supplied observations: B’s observed mean is 5 mm greater than A’s. It does not establish the mean for all three A cards, because A3 remains unknown. If A3 is measured later, retain this version and recompute with the new entry. Avoid describing B as generally wider across a larger population; the example supplies neither a sampling method nor evidence about any additional cards.