Methods
The study was approved according to the Economic and Social Research Institute’s (ESRI) ethics policy and was carried out in accordance with relevant guidelines and regulations. The ESRI granted a waiver of consent, due to the risk of bias in response rates and hence the validity of the findings. The trial design and analysis plan were pre-registered at Copies of all letters are available at the project’s Open Science Framework page: The “behaviourally informed” (BI) letter group (The “BI + envelope” group (The “BI + envelope + map” group (Letters were issued on March 9th and 10th 2023. The delivery failure rate was lower than expected at 13.3%. A chi-square test for differences in the delivery rate across groups was negative, χFlow diagram.The final anonymised dataset included data on the letter issued by the household, their county, the date the letter was issued, if it delivered successfully, if the household responded, the date of any response and the response mode.