Reading a Nursery's Water Report: What the Figures Will Not Say
Read the following passage from a community newsletter and answer the questions on it.
'Since the municipal nursery switched to drip irrigation in March, water use on its two-hectare site has fallen by 38%, according to its quarterly meter readings. The nursery's manager credits the change, but notes that March also brought the end of the municipal hosepipe ban and the start of the rainy season. Two of the four irrigation zones were replanted in April with younger, thirstier seedlings. Staff have also stopped washing the paving every Friday, a task that used to run for about forty minutes. The nursery's water bill, however, rose slightly, because the municipality raised tariffs by 12% in July. The manager says the figures prove that drip irrigation is the most effective water-saving measure the nursery has ever adopted.'
1. State what the passage does and does not allow a reader to conclude about the drip irrigation, and explain the difference.
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What it allows. Metered water use on the site was 38% lower in the quarter after the switch than in the quarter before it, as recorded by the meter. That is a fact about a measurement, and it is stated with its source ('quarterly meter readings'), which is good practice.
What it does not allow. That drip irrigation caused the fall, and certainly not that it is 'the most effective measure the nursery has ever adopted'. Five things changed in the same window, and the passage lists four of them: the hosepipe ban ended, the rainy season began, two of four zones were replanted, and the Friday paving wash stopped. Any of these could account for part or all of the reduction, and the replanting actually pushes in the opposite direction - younger seedlings usually need more water, which means the true saving from the drip system may be larger than 38% but cannot be isolated from the figures given.
The distinction that matters. The passage supports a before-and-after description, not a cause. A causal claim needs a comparison that holds the other factors constant - for example, metering one zone on drip against a similar zone still on sprinklers through the same months. Without that, 'since' is doing the work of 'because', and the two are not the same word.
2. The passage reports a 38% fall in water use and a 12% rise in tariffs, yet says the bill rose slightly. Test whether these three statements are consistent.
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Set up the arithmetic. If the 38% fall and the 12% rise applied to the same baseline, the new bill would be the old bill multiplied by the change in volume and the change in price:
0.62 × 1.12 = 0.694
That is a fall of about 31%, not a slight rise. A bill of N$ 10 000 would become about N$ 6 940.
So the three statements cannot all describe the whole account in the ordinary way. Something else is going on, and the passage does not say which:
- the 38% may refer to metered consumption at the site's irrigation meters only, while the bill also carries fixed charges, connection fees or a different metered supply;
- consumption outside the metered irrigation account may have risen;
- the 38% may compare a wet quarter with a dry one rather than like with like, which links straight back to the rainy-season point;
- the tariff rise may apply only from July, so it affects part of the billing period.
Why this matters to a reader. A writer who puts two percentages side by side invites the reader to combine them, and here the combination contradicts the stated outcome. That contradiction is not proof of dishonesty - it is proof that the passage has left out the one figure that would make the account coherent: the actual bills, or the volumes they were based on. A careful reader asks for those before repeating any of the three numbers.
3. Rewrite the manager's final sentence as a cautious statement that the passage could actually support, and explain each change you make.
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Rewrite. 'Metered water use on the site was 38% lower in the quarter after drip irrigation was installed than in the previous quarter. Other changes in the same period - the end of the hosepipe ban, the start of the rainy season, replanting in two zones and the cancellation of the Friday paving wash - may also have contributed, so the size of the saving attributable to drip irrigation has not yet been established.'
Change by change.
- 'Prove' → 'was ... lower'. The original asserts causation from a before-and-after comparison; the rewrite reports the measurement.
- 'The most effective measure ever adopted' → deleted. A superlative needs a comparison across all measures over all years. The passage offers one quarter and no comparison group, so the claim cannot be tested at all - which is worse than being wrong.
- 'Credits the change' → a list of rival explanations. Naming them is what makes the sentence honest; it shows the reader what has been weighed rather than asking them to trust the writer.
- 'Has not yet been established' → a hedge with a direction. This is not vagueness for its own sake: it says what is known, what is not, and implicitly what evidence would settle it (a metered comparison between zones).
- Tone. The rewrite is still favourable to drip irrigation - the 38% figure stands - but it no longer asks the reader to believe more than the evidence carries, which is the difference between a claim that survives scrutiny and one that collapses the moment a sceptic reads the meter readings.