Deposit-Return Scheme: Two Briefings, One Conclusion
Two briefings on the same deposit-return scheme reach a parliamentary committee on the same day. Read both and answer the questions.
Briefing A (retail association): 'Since the 15-cent deposit on plastic bottles was introduced in the (fictional) region of Astoria, sales of bottled water fell 9% and reported litter from plastic bottles fell 34%. Collection points, however, are concentrated in supermarkets; rural residents travel an average of 11 km to return a bottle. Retailers report a 6% rise in staff time at checkouts.'
Briefing B (environmental agency): 'Litter surveys in Astoria show a 34% fall in plastic bottles, but a 12% rise in littered glass and aluminium, which carry no deposit. Overall litter weight fell only 4%. The deposit has changed what is littered more than how much.'
1. Which claim is supported by both briefings, and which claims appear in only one?
Read the worked answer
Supported by both: plastic-bottle litter fell by 34%. Briefing A reports it as 'reported litter from plastic bottles fell 34%'; Briefing B reports it from 'litter surveys'. The figure and direction match, so the shared claim is safe even though the sources differ in method.
Only in A: the 9% fall in bottled-water sales; the 11 km average rural return distance; the 6% rise in checkout staff time.
Only in B: the 12% rise in glass and aluminium litter; the 4% fall in overall litter weight; the interpretive claim that the deposit changed composition more than volume.
This matters because a summary that says 'both briefings agree litter fell substantially' is misleading. They agree only on the plastic-bottle component; B explicitly qualifies the total with a 4% figure. Synthesis requires matching claims, not merging them.
2. A committee member concludes: 'The deposit scheme has failed to reduce litter.' Assess this using both briefings.
Read the worked answer
The word 'failed' is not supported, though the claim contains a grain of truth.
Evidence against 'failed':
- Plastic-bottle litter fell 34% (both briefings).
- Overall litter weight fell 4% (Briefing B) - small, but a reduction, not a failure.
Evidence that qualifies the success:
- Glass and aluminium litter rose 12%, so the composition shifted towards undeposited materials.
- The 4% overall figure shows the headline 34% overstates the total effect.
A fair verdict: the scheme achieved a targeted reduction in one material while displacing some litter to others, producing a modest net fall. 'Failed to reduce litter' contradicts the 4% figure. 'A complete success' contradicts the 12% rise. The member has taken the strongest single statistic from each briefing and ignored the rest - the opposite of synthesis.
3. What evidence would most strengthen the claim that the deposit, rather than something else, caused the 34% fall?
Read the worked answer
A comparison with a similar region that did not introduce a deposit, over the same period, showing plastic-bottle litter held steady there.
Why this is the strongest option: it addresses the central rival explanation - a concurrent anti-litter campaign, a change in reporting practice, or a general behavioural trend - by holding those influences roughly constant. If Astoria fell 34% while the comparison region did not, the deposit becomes the leading explanation.
Useful but weaker alternatives:
- A longer before-and-after series in Astoria: improves trend confidence but still lacks a control.
- Survey data on consumer behaviour: shows mechanism, not effect size.
- Retail sales data alone: the 9% fall in bottled-water sales helps, but litter can fall for reasons unrelated to purchases.
The test is whether the evidence isolates the cause or merely adds another correlation to the same event.