NEW UPDATE: Completed trials may now upload and register supplementary documents (e.g. null results reports, populated pre-analysis plans, or post-trial results reports) in the Post Trial section under Reports, Papers, & Other Materials.
Analyzing the effect of biodegradable packaging on plastic consumption in the wholesale market: a field experiment in the Costa Rican farmer’s market
Initial registration date
October 25, 2019
October 28, 2019 1:36 PM EDT
Other Primary Investigator(s)
Wageningen University and Research Center
University of Gothenburg
Additional Trial Information
Command and control instruments, such as plastic-bag bans, have been effective in countries with a robust enforcement mechanism. In developing countries, these instruments have been partially effective in the short term but with no long-time effects due to many factors such as weak enforcement and lack of compliance, especially in the wholesale market. The farmers market is an open market with high consumption of plastic bags, which are commonly given to the consumers for free as a courtesy, from which more than 50% end up in the landfill.
Literature shows that changes in behavior can be observed if alternative solutions are provided to consumers. In the specific case of plastic, biodegradable products could be an option to substitute single-use plastic, especially in environments where regulation is low. Farmers' markets in Costa Rica are well established and have specific guidelines and rules for its operation, representing an opportunity to work closely and test different strategies to reduce plastic consumption in this sector. The project's aim is to identify the effect of pricing and default options on consumption behavior when providing environmentally friendly alternatives in the wholesale market. We will implement a field experiment in 12 farmers' markets in Costa Rica. Three treatments will be implemented: 1) the provision of biodegradable and plastic bags for free, 2)the provision of biodegradable bags at a cost keeping the non-biodegradable bags free of charge; and, 3) a treatment where the biodegradable bags are the default option with an associated cost. We hypothesize that consumers will consume more biodegradable bags when they are provided free of charge, a rebound effect of introducing a more "environmentally friendly substitute." The impact of the default option with the price is anticipated to have a positive impact by reducing plastic consumption as a whole. We expect our results to show that more environmentally friendly alternative products free of charge will not necessarily reduce consumption. Therefore, strategies to reduce plastic pollution should be oriented to reduce current consumption patterns. Registration Citation
Lanza Castillo, Gracia María, Francisco Alpizar and Fredrik Carlsson. 2019. "Analyzing the effect of biodegradable packaging on plastic consumption in the wholesale market: a field experiment in the Costa Rican farmer’s market ." AEA RCT Registry. October 28.
The treatments are:
1. Treatment 1. Free plastic + free biodegradable: the vendor will offer biodegradable as an alternative mentioning the harmful impacts of non-biodegradable plastic bags. The intention is that the farmer read/express at loud the following message:
“Would you like to reduce pollution in the oceans by using a biodegradable plastic bag for your *name of the fruit/vegetable” instead of a non-biodegradable bag?”
To guarantee that the consumer has been exposed to the message we will install in each stands at the eye level a sign with the message.
2. Treatment 2. A price for biodegradable (25 colones) + free plastic: the price was set based on a previous willingness to pay survey among consumers. For those customers that request a bag, the vendor will offer the biodegradable bag for a cost of 25 colones, if the consumer decides not to pay then a non-biodegradable plastic bag will be offered to avoid any loss to the vendor. The message is the follows: “Would you like to reduce pollution in the oceans by using a biodegradable plastic bag for your *name of the fruit/vegetable” instead of a non-biodegradable bag?”, it cost you 25 colones per bag?”
3. Treatment 3. a price for biodegradable as a default option + free plastic”: the price was set based on a previous willingness to pay survey among consumers. For those customers that request a bag, the vendor will use the biodegradable bag as the default option, telling the consumer that it cost an extra 25 colones, if the consumer decides not to pay then the packaging of the product will change to a non-biodegradable plastic bag to avoid any loss to the vendor. We will use the same message as in treatment two.
Measurements will take place every day before the market begins and after 12:30 p.m. At the end of the two-week treatment period we will apply the KAP survey to the vendors. We will hire one enumerator per farmer market; each enumerator will have training on how to apply the survey and perform measurements. Each enumerator will have a digital kitchen scale with grams precision. Signs about biodegradable bags will be located at the entrance of each market for all treatments. Also, we have set a list of observation criteria that the enumerator has to pay attention to every day of the market to control for other external factors, such as weather, any municipal activity in the district, school activities, fairs, etc. Although we are interested in changes in individual consumer behavior, it is not possible to measure individual consumption; and therefore, we will measure the amount of plastic delivered via the vendor in the market. Notwithstanding, we will conduct a consumer survey during the baseline data collection (after all instruments in the first stage have been collected) to explain initial levels of plastic bag consumption. The survey includes some questions from the KAP and specific questions related to knowledge and perception towards biodegradable packaging.
Intervention Start Date
Intervention End Date
Primary Outcomes (end points)
1. The difference in the amount of non-biodegradable plastic bags used in the farmer market after the treatment: the project will provide the plastic bags to 15 farmers in each farmer market and at the end of the day we will count how many plastic bags were used.
2.The number of biodegradable plastic bags used during the farmer’s market during the treatments, to observe any substitution effect and rebound effect.
3. The number of non-biodegradable plastic bags used in the baseline.
Primary Outcomes (explanation)
Secondary Outcomes (end points)
Secondary Outcomes (explanation)
The experiment follows a between-subject design in a cluster-randomized setting. It involves tablet-based surveys in 2 stages. The first stage considers the application of three instruments:
1) Knowledge, Attitudes, and Practices (KAP) survey for the vendor: the KAP aims to capture information about the current knowledge with regards to marine plastic pollution, attitudes towards general environmental problems and the marine plastic pollution in particular; and, finally it elicit stated behavior towards consumption of plastic. This survey has been designed to be apply to the vendors of the fruits and vegetables. 2) The vendor profile: the aim of this survey is to collect background information about the vendor/farmer in terms of the type of products that he sells in the farmers market, number of family members involved in the activity, current perception towards biodegradable bags, perception towards solutions to plastic pollution, perception with regards to his consumers, sources of additional income.
3) Measurement instrument: this is the basic instrument to be used before and after to measure the number of plastic bags used during the entire research project.
The three instruments will be conducted during a two-week baseline survey to record the number of plastic bags consumed by each vendor per day from 5:00 a.m. to 12:30 p.m. The first survey to be applied is the KAP survey, followed by the measurement instrument and last the vendor profile. All surveys followed the xlsform, using the ONA.IO server and the ODK collect app for android. Measurements will take place every day before the market begins and after 12:30 p.m.
We propose a cluster sampling, where each farmer's market is a cluster with a total of 15 farmers per cluster. In total, we have 12 clusters, six markets from Heredia, and six markets from Cartago. Farmers will be selected randomly from a list of vendors provided by the board administering the wholesale market. The selection will be among those farmers that sell fruit and vegetables, which use the highest volume of ultra-thin plastic bags with very small micron sizes. Specific vendors of products such as processed food (cheese, juices, etc.) are excluded from the experiment. Preliminary work identified that the plastic bags more commonly used in the market are 9x14 and 10x26 inches; and, that on average, each farmer utilizes 819 plastic bags (or 2.15kg) of a 9x14 inches per day. We will focus on changes in the consumer demand for plastic bags with the size of 9x14 inches. Pilot data
To evaluate the understanding of the survey instrument, we will conduct a pilot study with 15 farmers from a farmer wholesale market.
Experimental Design Details
Following (Coville 2013) and using STATA 15, we assigned the treatments randomly. Under the random assignment, treatment exposure is statistically independent of any (measured or unmeasured) factor that might also affect outcomes (Wing, Simon, and Bello-Gomez 2018).
Was the treatment clustered?
Sample size: planned number of clusters
3 cluster for control, 3 clusters for treatment 1, 3 clusters in treatment 2, 3 clusters for treatment 3. in total 12 farmers market
Sample size: planned number of observations
15 farmers per market, 180 in total.
Sample size (or number of clusters) by treatment arms
45 control, 45 treatment 1, 45 treatment 2, 45 treatment 3.
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)
With three clusters per treatment and 15 farmers per cluster, we achieve a power of 89%, anticipating a reduction in the number of plastic bags consumed by 35%. The 35% is conservative, considering that the literature reports a decrease of more than 50% when pricing is introduced.
INSTITUTIONAL REVIEW BOARDS (IRBs)
Environment for Development
IRB Approval Date
Post Trial Information
Is the intervention completed?
Is data collection complete?