Encouraging householders to separate compostable organic waste from general landfill waste is a primary target for environmental psychologists. One cognitive intervention involves using descriptive social norms to nudge behavior. An environmental psychologist investigated whether placing a visual prompt stating "90% of residents in your area recycle food waste" on kitchen trash cans would reduce the mass of compostable food waste incorrectly discarded in them.
A sample of 12 households was selected using opportunity sampling. In the baseline week, participants recorded the mass (in kilograms) of organic waste incorrectly thrown into their general landfill bin. The following week, the social norm cognitive prompt was affixed to the lid of each household's landfill bin, and the mass of incorrectly discarded organic waste was recorded again. The collected data are displayed in the table below:
| Household | Weekly mass of organic waste incorrectly discarded in general landfill bin (kg) | |
|---|---|---|
| Without cognitive prompt | With social norm cognitive prompt | |
| A | 8.4 | 5.1 |
| B | 12.1 | 8.4 |
| C | 6.5 | 4.2 |
| D | 14.2 | 9.8 |
| E | 9.8 | 6.5 |
| F | 11.3 | 8.1 |
| G | 5.2 | 3.0 |
| H | 13.0 | 9.2 |
| I | 7.7 | 4.9 |
| J | 10.6 | 7.3 |
| K | 6.9 | 4.5 |
| L | 9.1 | 6.0 |
Calculate the mean mass of organic waste incorrectly discarded in the general landfill bin in the week with the social norm cognitive prompt. Show your workings and write your final answer to three significant figures.