Encouraging individuals to recycle more effectively is a key challenge in environmental psychology, with researchers often focusing on cognitive prompts to interrupt habitual behaviors. An environmental psychologist investigated whether placing a reflective cognitive prompt card (reading "Stop & Think: Is this recyclable?") onto the lid of a household's general waste bin would reduce the amount of recyclable waste incorrectly discarded in it.
She conducted an experiment using a sample of 12 households obtained through opportunity sampling. For one week, participants recorded the number of dry recyclable items (plastic bottles, cardboard packaging, tin cans, etc.) incorrectly thrown into their general waste bin in-between collections. The following week, the reflective cognitive prompt card was attached to the lid of the general waste bin, and the same households again recorded the number of recyclable items incorrectly discarded inside it. The data is presented in the table below.
| Household | Number of recyclable items thrown into general waste bin during one week | |
|---|---|---|
| Without cognitive prompt card | With cognitive prompt card | |
| A | 18 | 13 |
| B | 31 | 24 |
| C | 14 | 10 |
| D | 25 | 18 |
| E | 22 | 19 |
| F | 29 | 28 |
| G | 12 | 7 |
| H | 35 | 26 |
| I | 19 | 14 |
| J | 27 | 21 |
| K | 16 | 15 |
| L | 20 | 14 |
Calculate the mean for the number of recyclable items thrown into the general waste bin in the week with the cognitive prompt card. Show your workings and write your answer to three significant figures.