Effects of Incentives vs Normative Feedback on Bio-waste Sorting in the Field

INTRODUCTION: The treatment and randomization unit in the study will be households located in the city of Palma (Spain) where individuals must use a personal card to use the bins meant for bio-waste sorting. Hence, each bin use is recorded and associated with one individual, allowing us to track the participants' use of the bio-waste sorting bins.

RECRUITMENT: Participants were recruited between the 1st of March 2023 and lasted until the 30th of June. Recruitment was carried out face-to-face by directly approaching people in the street, by the internet through advertisements in social media, and by placing flyers in the city. The participants were informed that the study would be about their bio-waste sorting behavior and that they would receive messages by WhatsApp; recruitment was facilitated by offering those who signed up to win one of four prizes of €250 each (only one individual per household could sign up). The certified random draw of the winners was performed by the independent firm "Easypromos" on the 27th of July. Each participant signed an informed consent form and answered a questionnaire that collected basic demographic data (number and ages of people in the participant's household, annual income, contact information, …) as well as data on social norms, personal norms, and relationship with nature (beliefs about average environmental sensitivity in the population, motivations to recycle, ...). Among those who signed up, only the citizens of Palma who have the card required to open the bio-waste sorting bins were included in the final sample.

ALLOCATION TO TREATMENTS: The 1620 participants were divided into three groups of equal size. The three groups are balanced in terms of the recruitment process.

INTERVENTION, FIRST WAVE: We let the sample rest until October 17th, 2023. At that moment, we record the number of weeks that each participant used a bio-waste sorting bin at least once, during the five weeks between September 4th and October 8th 2023 (henceforth, this number between 0 and 5 is the individual's "past bio-waste sorting behavior"). In the week of October 17th 2023, all the participants will receive WhatsApp messages with information about the effects of their actual bio-waste recycling behavior. To build the information on the effects derived from this behavior, we first present them with information about the average amount of CO2 emissions reduction that a citizen in Palma achieves by sorting the bio-waste they produce in one week, as follows (this number was calculated based on past bio-waste sorting data provided to us by the waste management company Emaya):

"Do you know that every week that a citizen of Palma recycles their biowaste, they reduce CO2 emissions by, approximately, 1 kg? This is the same amount of CO2 that a tree absorbs by a tree in about 17 days."

We then exploit variations in which information is presented to the participants to answer our research question, the three treatments being:

1. CONTROL: each participant receives information about their past bio-waste sorting behavior (see above), as follows:

a. If their past bio-waste sorting behavior is zero, they receive the following message:
"During the 5 weeks between 04/09/2023 and 08/10/2023, your household used the biowaste bin for 0 weeks. This means that, during this period, each member of your household reduced CO2 emissions by 0 kg, out of the 5kg reduction that could have been achieved. A tree needs 85 days to absorb 5kg of CO2."

b. If their past bio-waste sorting behavior is some number Y between 1 and 5, they receive the following message:
"During the 5 weeks between 04/09/2023 and 08/10/2023, your household used the biowaste bin for Y weeks. This means that, during this period, each member of your household reduced CO2 emissions by Y kg, out of the 5kg reduction that could have been achieved. A tree needs 17Y days to absorb Y kg of CO2."

2. COMPARISON: each participant receives the same information as those in the CONTROL treatment, and they also receive information about other households' past bio-waste sorting behavior:

"In the same period, the other households in the study used the biowaste bin for an average of 0.73 weeks. This means that, during this period, each member of those households reduced CO2 emissions by 0.73 kg, out of the 5kg reduction that could have been achieved. A tree needs 12 days to absorb 0.73 kg of CO2."

3. KANTIAN: each participant receives the same information as those in the CONTROL treatment, and they also receive the following message:

"Big changes are the sum of small efforts. Imagine by how much CO2 emissions would be reduced if your household recycled its biowaste, and all the other households in Palma also did the same as yours. A large number of trees would be needed to achieve the same reduction."

INTERVENTION, SECOND WAVE: The same messages will be sent a second time, based on observed behaviors during the five weeks between January 8th and February 11th 2024 (this period may have to be slightly adjusted depending on unforeseeable events; the objective is to avoid potentially strong seasonal effects around the Christmas, New Year's and Reyes celebrations). Messages would be sent one week after closing the “data entry”.

TIME HORIZON OF THE STUDY: To evaluate the effects of the intervention in the medium-long term, we will wait for a minimum of 3 months to start running the analysis. Data on the participants' bio-waste bin use is collected for every week following the date they signed up, and until July 2024.

OTHER INFO/COMMENTS:
1. The last week of February and the first of March 2023, we conducted a lab-experiment designed to test the effects of several messages. This lab-experiment was used to tailor the field intervention.
2. If feasible, after the intervention (in June or July 2024), we will run a post-intervention survey to collect information about the mechanisms at work.

2023-10-17

2024-06-30

- Effect of Wave 1 messages: proportion of weeks that each household uses the bio-waste sorting bin at least once between October 23rd 2023 and February 11th 2024 (Data of the week in which the households receive the WhatsApp message is not analyzed).
- Effect of Wave 2 messages: proportion of weeks that each participant uses the bio-waste sorting bin at least once between February 19th 2024 and the end date (in June or July 2024).

- Effect of Wave 1 messages: average number of bio-waste sorting bin openings per week, between October 23rd 2023 and February 11th 2024 (Data of the week in which the households receive the WhatsApp message is not analyzed).
- Effect of Wave 2 messages: average number of bio-waste sorting bin openings per week, between February 19th 2024 and the end date (in June or July 2024).

This RCT consists of a field experiment about bio-waste recycling in the city of Palma (Spain). Citizens of Palma must scan a card that identifies them in order to use the bins meant for bio-waste sorting. Such bins are available on streets throughout the city, next to the bins for recycling of glass, plastic, and paper. The city's waste management company will provide the research team with data on the bio-waste sorting behaviors of the 1620 households who signed up to participate in the study between February and July 2023. The goal is to study how the participants' use of the bio-waste sorting bins is affected by feedback. The participants will be allocated to one of three treatments. Each participant will receive a first round of feedback messages between October 17 and 20, and then a second round of messages at some point in February 2024.

TREATMENTS, FIRST WAVE: We let the sample rest until October 17th, 2023. At that moment, we record the number of weeks that each participant used a bio-waste sorting bin at least once, during the five weeks between September 4th and October 8th 2023 (henceforth, this number between 0 and 5 is the individual's "past bio-waste sorting behavior"). In the week of October 17th 2023, all the participants will receive WhatsApp messages with information about the effects of their actual bio-waste recycling behavior. To build the information on the effects derived from this behavior, we first present them with information about the average amount of CO2 emissions reduction that a citizen in Palma achieves by sorting the bio-waste they produce in one week, as follows (this number was calculated based on past bio-waste sorting data provided to us by the waste management company Emaya):

"Do you know that every week that a citizen of Palma recycles their biowaste, they reduce CO2 emissions by, approximately, 1 kg? This is the same amount of CO2 that a tree absorbs by a tree in about 17 days."

We then exploit variations in which information is presented to the participants to answer our research question, the three treatments being:

1. CONTROL: each participant receives information about their past bio-waste sorting behavior (see above), as follows:

a. If their past bio-waste sorting behavior is zero, they receive the following message:
"During the 5 weeks between 04/09/2023 and 08/10/2023, your household used the biowaste bin for 0 weeks. This means that, during this period, each member of your household reduced CO2 emissions by 0 kg, out of the 5kg reduction that could have been achieved. A tree needs 85 days to absorb 5kg of CO2."

b. If their past bio-waste sorting behavior is some number Y between 1 and 5, they receive the following message:
"During the 5 weeks between 04/09/2023 and 08/10/2023, your household used the biowaste bin for Y weeks. This means that, during this period, each member of your household reduced CO2 emissions by Y kg, out of the 5kg reduction that could have been achieved. A tree needs 17Y days to absorb Y kg of CO2."

2. COMPARISON: each participant receives the same information as those in the CONTROL treatment, and they also receive information about other households' past bio-waste sorting behavior:

"In the same period, the other households in the study used the biowaste bin for an average of 0.73 weeks. This means that, during this period, each member of those households reduced CO2 emissions by 0.73 kg, out of the 5kg reduction that could have been achieved. A tree needs 12 days to absorb 0.73 kg of CO2."

3. KANTIAN: each participant receives the same information as those in the CONTROL treatment, and they also receive the following message:

"Big changes are the sum of small efforts. Imagine by how much CO2 emissions would be reduced if your household recycled its biowaste, and all the other households in Palma also did the same as yours. A large number of trees would be needed to achieve the same reduction."

TREATMENTS, SECOND WAVE: The same messages will be sent a second time, based on observed behaviors during the five weeks between January 8th and February 11th 2024 (this period may have to be slightly adjusted depending on unforeseeable events; the objective is to avoid potentially strong seasonal effects around the Christmas, New Year's and Reyes celebrations). Messages would be sent one week after closing the “data entry”.

Household level randomized by a computer (randtreat command in Stata, with setseed(15)). Stratified by recruitment source (Form, Twitter, street, other)

The randomization unit is the household.

No

1620 households.

Weekly behavior of the 1620 households.

540 households per treatment arm.

According to data collected in previous studies (see Alonso-Paulí et al., 2023), an average household in the city of Palma uses bio-waste sorting bins with a weekly frequency of 0.29, showing a standard deviation of 0.354. For a power analysis of 80%, given the sample size we intend to have, we guarantee, at least, a minimum detectable effect of 0.0767 in the use frequency (at 95% confidence).