metadata: authors: 0: author: Dissanayake,Sahan T. M.
1: author: Voigt,George
2: author: Cooper, Abbie
3: author: Beyene, Abebe Damte
4: author: Bluffstone,Randall
5: author: Gebreegziabher,Zenebe
6: author: LaFave, Daniel
7: author: Martinsson,Peter
8: author: Mekonnen,Alemu
9: author: Toman,Michael A.
count: Ethiopia
docna: 0: docna: Stability and Evolution of Preferences for
            Improved Cookstoves -- A Difference-in-Difference Analysis
            of a Choice Experiment from Ethiopia
docty: Policy Research Working Paper
subtopic: Global Environment,Flood Control,Hydrology,Energy Demand,Energy and Environment,Energy and Mining,Health Care Services Industry
teratopic: Industry,Water Resources,Energy
abstracts: cdata!: There is a growing effort in the
            non-market valuation literature toward better understanding
            of the stability and evolution of preferences over time. The
            study uses a novel approach combining a repeated choice
            experiment with a randomized controlled trial on stove
            adoption in Ethiopia to analyze the stability and evolution
            of preferences. The treatment group in the randomized
            controlled trial received an improved fuelwood stove with
            less fuelwood use, whereas the control group continued to
            use traditional cooking methods. Respondents were given the
            exact same choice questions in 2013 and 2016. The study
            began with 504 households in 36 communities in 2013, and 486
            of the same households participated in 2016 (a 96 percent
            retention rate). The results show that preferences of the
            respondents from the control group are stable over the study
            period, while preferences of the respondents from the
            treatment group evolve. Moreover, households in the
            treatment group still using the stoves have significantly
            higher willingness to pay for all the stove's
            attributes in 2016 compared with 2013, indicating how longer
            experience can increase the willingness to pay for
            technology with environmentally preferable attributes.
geo_reg_key: 517191
repnme: repnme: Stability and Evolution of Preferences for
            Improved Cookstoves -- A Difference-in-Difference Analysis
            of a Choice Experiment from Ethiopia
display_title: Stability and Evolution of
            Preferences for Improved Cookstoves -- A
            Difference-in-Difference Analysis of a Choice Experiment
            from Ethiopia
listing_relative_url: /research/2019/06/31214405/stability-evolution-preferences-improved-cookstoves-difference-in-difference-analysis-choice-experiment-ethiopia
topicv3: Water m1326522 137,Energy and Extractives m1326595 507,Environment, Natural Resources and Blue
            Economy m1327391 566,Flood Control m1503320 3221,Health m1504661 3480
publishtoextweb_dt: 2019-06-28T00:00:00Z
historic_topic: Industry,Water Resources,Energy
no_of_pages: 18
author_keys: 0: author_key: 166060
model_extractions: Policy Research Working Paper 8928

# Stability and Evolution of Preferences for Improved Cookstoves

A Difference-in-Difference Analysis of a Choice Experiment from Ethiopia _Sahan T. M. Dissanayake George Voigt Abbie Cooper Abebe Damte Beyene Randall Bluffstone Zenebe Gebreegziabher Daniel LaFave Peter Martinsson Alemu Mekonnen Michael Toman_ Development Economics Development Research Group June 2019
Policy Research Working Paper 8928

# **Abstract**

There is a growing effort in the non-market valuation literature toward better understanding of the stability and evolution of preferences over time. The study uses a novel approach combining a repeated choice experiment with a randomized controlled trial on stove adoption in Ethiopia to analyze the stability and evolution of preferences. The treatment group in the randomized controlled trial received an improved fuelwood stove with less fuelwood use, whereas the control group continued to use traditional cooking methods. Respondents were given the exact same choice questions in 2013 and 2016. The study began with 504 households in 36 communities in 2013, and 486 of the same households participated in 2016 (a 96 percent retention rate). The results show that preferences of the respondents from the control group are stable over the study period, while preferences of the respondents from the treatment group evolve. Moreover, households in the treatment group still using the stoves have significantly higher willingness to pay for all the stove’s attributes in 2016 compared with 2013, indicating how longer experience can increase the willingness to pay for technology with environmentally preferable attributes.

This paper is a product of the Development Research Group, Development Economics. It is part of a larger effort by the World Bank to provide open access to its research and make a contribution to development policy discussions around the world. Policy Research Working Papers are also posted on the Web at http://www.worldbank.org/prwp. The lead author and task team lead leader may be contacted at sdissan2@gmail.com and mtoman@worldbank.org.

_The Policy Research Working Paper Series disseminates the findings of work in progress to encourage the exchange of ideas about development issues. An objective of the series is to get the findings out quickly, even if the presentations are less than fully polished. The papers carry the names of the authors and should be cited accordingly. The findings, interpretations, and conclusions expressed in this paper are entirely those of the authors. They do not necessarily represent the views of the International Bank for Reconstruction and Development/World Bank and its affiliated organizations, or those of the Executive Directors of the World Bank or the governments they represent._ Produced by the Research Support Team
# **Stability and Evolution of Preferences for Improved Cookstoves - A Difference-inDifference Analysis of a Choice Experiment from Ethiopia**

Sahan T. M. Dissanayake<sup>*</sup> , George Voigt, Abbie Cooper, Abebe Damte Beyene, Randall Bluffstone, Zenebe Gebreegziabher, Daniel LaFave, Peter Martinsson, Alemu Mekonnen, Michael Toman

**Keywords:** temporal stability, preferences, improved cookstoves, choice experiment, Ethiopia

**JEL Codes:** Q51, Q56, D00

> * Corresponding author: Dissanayake, Portland State University, Portland, OR, USA <u>sdissan2@gmail.com</u> Dissanayake: Portland State University, Portland, OR, USA; Beyene: Ethiopian Development Research Institute, Addis Ababa, Ethiopia; Cooper: Colby College, Waterville, ME, USA; Gebreegziabher, Ethiopian Development Research Institute and Mekelle University, Addis Ababa and Mekelle, Ethiopia; LaFave: Colby College, Waterville, ME, USA; Martinsson, P: Gothenburg University, Gothenburg, Sweden; Mekonnen, Ethiopian Development Research Institute and Addis Ababa University, Addis Ababa, Ethiopia; Toman, The World Bank, Washington DC, USA; Voigt: Colby College, Waterville, ME, USA. Financial support for this research from the World Bank and from the Knowledge for Change Trust Fund is gratefully acknowledged.
# **Stability and Evolution of Preferences for Improved Cookstoves - A Differencein-Difference Analysis of a Choice Experiment from Ethiopia**

# **1. Introduction**

Non-market valuation methods are used to determine the value, preferences and potential consumer demand for non-market environmental goods and policies. Valuation studies, in particular statedpreference methods that involve surveys, are often costly, and therefore conducted infrequently (Bliem et al. 2012, Brouwer and Bateman 2005, Mørkbak and Olsen 2014, Price et al. 2016). If the values obtained from these studies are stable over time, meaning there is no significant change in preferences among the given survey sample over time, they can be used in future analysis (Bliem et al. 2012, Brouwer and Bateman 2005, Morten and Olsen 2014, Price et al. 2016). Therefore, rather than acquiring funding for data collection, historical data that exhibit stability can be used for benefit transfer and cost-benefit analysis in the future.

However, due to the hypothetical nature of these valuation techniques, it is often difficult to determine the credibility or validity of the results as time progresses. Credibility is a measure of temporal validity, or as Mørkbak and Olsen (2014) state, “Does an individual respondent provide identical answers to identical questions over time, assuming that preferences are stable?” If preferences are not found to be stable, the use of results from stated preference studies conducted in the past for benefit transfer, cost-benefit analysis and calculation of welfare over time becomes problematic.

Further, the few studies on measuring stability of preferences elicited from choice experiment surveys have been conducted in developed country settings. At the same time there is a growing use of non-market valuation surveys, especially choice experiments, in developing country contexts to guide environmental policy and regulations. Understanding if preferences elicited in developing country settings, especially in rural communities, are stable is important with regard to using results from non-market valuation studies over time.

Therefore, the first focus of this work is to determine if preferences elicited using choice experiment surveys in developing country settings are stable over time. We test the temporal stability of preferences for identical respondent groups over a three-year period, using identical choice experiment surveys, in a test-retest experimental setting.

The background of the study is a valuation study on preferences for clean cookstoves and a randomized controlled trial (RCT) of improved cookstoves conducted in Ethiopia. The main objective of the valuation study was to use a choice experiment survey to examine Ethiopian households’ 2
valuation of attributes of cookstoves, including durability, fuel reduction, smoke reduction, and amount of time saved by using new clean, cooking technology. The overall goal of the survey was to determine features of clean cookstoves that were well-liked by communities in order to help implement sustainable cooking practices in rural Ethiopia, with the hopes of reducing the harmful health and environmental impacts of traditional cooking methods. We use the choice experiment survey on preferences for attributes of clean cookstoves conducted in Ethiopia in 2013 and repeated in 2016 to analyze the temporal stability of preferences.

The unique setting of having a repeated choice experiment and an RCT allows us to also study technology adoption and how preferences and willingness-to-pay for improved cookstoves evolve with use and experience. There is a large economics literature on technology adoption (see Marra et al. 2003 and Conley and Udry 2010) and a smaller but growing literature on adoption and learning with regard to cookstoves (see Ruiz-Mercado 2011 and Miller and Mobarak 2014).

At the same time, it has been found that the Mirt stoves save fuelwood (Gebreegziabher et al., 2018) and that it may reduce household air pollution (Bluffstone et al. 2018). Therefore, the second focus of this work is to evaluate the degree to which such experience with such outcomes causes users to shift their preferences and change their demand for attributes.

Therefore, our research questions become _Q1: Are the preferences for cookstoves elicited from choice experiments stable over time?_ _Q2: How do users’ preferences and willingness to pay for cookstove attributes evolve after extensive experience with the Mirt stoves?_ The combined RCT and repeated choice experiment setting allows us to use a difference-indifference framework to analyze the results from the choice experiment, the first instance of a difference-in-difference choice experiment analysis that we are aware of. We use the control group to evaluate stability of preferences and the treatment group to evaluate the degree to which experience with the Mirt stove shifted user preferences and attribute values.

Results show that the willingness to pay for stove attributes of the control group are stable over the three-year period from 2013 to 2016 and that the preferences of the treatment group have evolved over this time period. To better understand how long-term stove use impacts preferences, we then refine the treatment group to only include those currently using the stoves and we find that, compared to the control group, long-term stove use increases the WTP for most of the stove attributes.

3
# **2. Literature on Stability of Preferences**

There are a small number of studies that examine the temporal stability of contingent valuation and choice experiment studies but the results on the stability of preferences are mixed. Brouwer and Bateman (2005) studied the temporal stability of WTP responses from two contingent valuation studies (on flood control and wetland conservation) administered five years apart. They find that when examining WTP values over an extended time period, there is a statistically significant decrease in how people value flood control and wetland conservation attributes over time. They attribute these results to the five-year gap in the survey administration, which is much longer than the test-retest period that most researchers use, and also to the utilization of expanded models to include more, ad hoc transitory factors.

Price et al. (2016) and Bliem et al. (2012) find differing results from Brouwer and Bateman (2005) in their analysis of the stability of WTP values obtained from stated preference surveys regarding the health effects of tap water consumption. Price et al. (2016) conducted their survey in 2004 and again in 2012 using multiple survey formats. Their findings show that there is no significant difference in WTP between the two survey formats, and they support the transfer of non-market benefits over extended time periods. Bliem et al. (2012) test for the temporal stability of individual preferences by using two identical surveys for a restoration project of a stretch of the Danube River in Austria repeated over a one-year period. The researchers find that the preferences are stable across the one-year time frame.

Mørkbak and Olsen (2014) test the reliability and stability using an incentivized test-retest choice experiment. In their study, they use a choice experiment for a market good with real economic incentives, and sample respondents using the exact same questionnaire twice within a two-week lag time. Analysis of the data shows that error variances between the two identical surveys do not differ significantly, but the larger the utility difference in a choice task, the larger the probability that the respondent will choose the same alternative in the retest. Morten and Olsen also determine that the longer the time a respondent takes to answer the choice sets in the retest, the lower the probability that they will choose the same alternatives in the retest as they did in the original test.

Our study assesses how households in Ethiopia value different attributes of improved cookstove technology, and if these values are stable over a period of three years. This work contributes to the existing body of work on the stability of WTP values elicited from CE surveys using a test4
retest structure and differs from previous studies in that a randomly selected portion of the sample, the RCT treatment group, received an actual improved Mirt stove after completing the survey in 2013. Therefore, when the same sample was given an identical survey in 2016, a portion of them had already experienced cooking with the new, efficient stove technology. Results from this study not only contribute to the growing body of research about temporal stability of preferences, but also contribute to the existing literature about global sustainable development through the adoption of clean technologies.

# **3. Empirical Approach**

Given the improved cookstove RCT and the repeated choice experiments that were conducted in 2013 and in 2016, our data allow us to study temporal stability and the evolution of preferences elicited using a choice experiment survey within a difference-in-difference framework. The choice experiment in 2013 was conducted immediately prior to the improved stove RCT.

## _Choice Experiment Design_

Choice experiments are a stated preference valuation method used to determine individuals’ willingness to pay (WTP) for goods that do not have attainable market values. A choice experiment survey was chosen for this study over other stated preference valuation methods because they provide information about individual attributes of the good being valued.

We follow standard practice in the choice modeling literature for this research (Adamowicz et al. 1997, Adamowicz et al. 1998, Carlsson and Martinsson 2003, Louviere et al. 2000). We initially conducted informal focus groups, engaged in discussions with researchers and then finally conducted 15 formal focus groups and a field trial of the survey. Through this process we identified five attributes that we use for the final choice experiment; durability of the stove, the reduction of fuel use, the reduction of smoke, decrease in cooking time, and the cost of the stove. The attribute levels are presented in Table 1.

5
|**Attributes**|**Description**|**Levels**|
|---|---|---|
|Durability of stove|How long the stove would be<br>functional for.|-<br>No improved stove<br>-<br>1-5 years<br>-<br>6-10 years<br>-<br>11-15years|
|Reduction in amount of<br>fuel wood used|The reduction in fuel wood use in<br>comparison to using the<br>traditional cookstove.|-<br>No reduction<br>-<br>25% reduction<br>-<br>50% reduction|
|Reduction of smoke|The reduction of smoke as a<br>result of using an improved stove.|-<br>No reduction<br>-<br>25% reduction<br>-<br>50% reduction|
|Amount of cooking time<br>reduction|The amount of cooking time<br>reduced by adopting an improved<br>stove.|-<br>No reduction<br>-<br>25% reduction<br>-<br>50% reduction|
|Cost of improved stove|The amount paid to acquire a new<br>improved stove.|-<br>No payment<br>-<br>100 Ethiopian Birr<br>-<br>200 Ethiopian Birr<br>-<br>300 Ethiopian Birr<br>-<br>400 Ethiopian Birr|

Table 1: Attribute Levels The survey presents a number of choice questions where each choice question consists of two stove alternatives that contain varied levels of the five stove attributes that a respondent can choose from, and an additional status quo option of no improved cookstove at no cost.<sup>1</sup> To account for learning, the first and seventh questions of the survey were identical, and the first question was dropped for data analysis (Carlsson, Mørkbak, and Olsen 2012). Figure 1 shows an example of one choice experiment question.

> 1 It is important to note that for the RCT treatment group, the status-quo option reflects going back to a traditional stove. We discuss how this might impact the interpretation of the results in the conclusion.

6
Choice experiment for stove adoption <!-- Start of picture text --> VerNo:_l_ QNa:_1l_<br>Attributes Alternative 1 Alternative 2 Status Quo<br>Durability of<br>stowe 1-5 years 6-10 years No improved stove<br>Amount of<br>cooking time _» 40 . 2 " -<br>"| |MBO | |MOOG || QOOG<br>50% reduction 25% reduction No reduction<br>Reduction in<br>amountfuel woodof —Oe:== Oe= —te==SS aS= Petf =}Se = tf =<br>50% reduction 25% reduction No reduction<br>Reduction of<br>smoke ; . " ask, ask, ast,<br>A, A, A,<br>50% reduction 25% reduction No reduction<br>Cast of = Ea!<br>improved mul‘ie a<br>stove Ea<br>= Ea “a No payment but no<br>i rel improved stove<br>100 birr 300 birr<br>Please<br>tick/mark (v)<br>only one<br><!-- End of picture text -->
i_— Pasc13 13 + ert5 k,13 k,13 + Bcost,13 13 + Pasc.16c 16c + 5k=1 k,16c k,16c + Beost,16c 16c + Pasc,16t 16t + e=15 k,16t K,16t + cost,16t COSt16t + €; ASC,16t k,16t cost,16t ASC,16c k,16c cost,16c . — ~~fi~~ t Boost
group remain stable for both categories of treatment households (except for stove durability for the second sample). The ASC estimates (Table 2) for the control households are not significant, indicating that the overall preferences for the stoves have not changed for control households. This conclusion is robust across both definitions of treatment households.

The standard deviation estimates from the MMNL indicate that there is significant heterogeneity for most attributes in both the treatment group and the control group for both definitions of treatment households. Given the complexity of the model that we are estimating, we are not able to include interaction terms to better analyze heterogeneity in the estimates.

The average WTP results for the base/2013 and the 2016 treatment households are provided in Table 4. We find that households in the treatment group are WTP on average 54 birr (about 2 USD) for a stove that lasts a year longer (17 birr in 2013), 86 birr (about 3 USD) for a stove that reduces cooking time by 10% (47 birr in 2013), 142 birr (about 5 USD) for a stove that reduces fuelwood use by 10% (62 birr in 2013), and 112 birr (about 4 USD) for a stove that reduces smoke by 10% (45 birr in 2013).

9
Table 2: Diff-in-Diff Results – Mean Estimates

||Mean Es<br>|timates<br>|SD Estim<br>|ates<br>|
|---|---|---|---|---|
||(1)|(2)|(3)|(4)|
|Treatment Defined As|<br>All Households in<br>|<br>Households Still<br>|<br>All Households in<br>|<br>Households Still<br>|
||Treatment Group|UsingMirt|Treatment Group|UsingMirt|
|_2013 Baseline Values_|||||
|ASC|25.79<sup>***</sup>|17.53<sup>***</sup>|-12.83<sup>***</sup>|-7.475<sup>***</sup>|
||(4.874)|(3.364)|(2.430)|(1.398)|
|StoveDurability|0.0597<sup>***</sup>|0.0608<sup>***</sup>|-0.0237|-0.0301<sup>*</sup>|
||(0.00876)|(0.00915)|(0.0194)|(0.0170)|
|TimeReduction|<br>0.0155<sup>***</sup>|<br>0.0166<sup>***</sup>|<br>0.00283|<br>0.00869<sup>**</sup>|
||(0.00209)|(0.00221)|(0.00593)|(0.00414)|
|FuelReduction|0.0207<sup>***</sup>|0.0221<sup>***</sup>|0.0242<sup>***</sup>|0.0290<sup>***</sup>|
||(0.00226)|(0.00247)|(0.00271)|(0.00301)|
|SmokeReduction|0.0143<sup>***</sup>|0.0163<sup>***</sup>|0.0122<sup>*</sup>|-0.0154<sup>**</sup>|
||(0.00356)|(0.00377)|(0.00697)|(0.00607)|
|Cost|-0.00333<sup>***</sup>|-0.00348<sup>***</sup>|0.00507<sup>***</sup>|0.00612<sup>***</sup>|
||(0.000663)|(0.000707)|(0.000656)|(0.000713)|
|_2016 Treatment Group relative to 2_|_013 Baseline_||||
|ASCxtreatment16|-2.324<sup>*</sup>|36.03|-1.530|4.435|
||(1.305)|(1259604.0)|(1.084)|(652309.5)|
|StoveDurabilityxtreatment16|<br>0.0994<sup>***</sup>|<br>0.141<sup>***</sup>|<br>0.192<sup>***</sup>|<br>0.247<sup>***</sup>|
||(0.0226)|(0.0317)|(0.0262)|(0.0351)|
|TimeReductionxtreatment16|0.0108<sup>**</sup>|0.0145<sup>**</sup>|0.0510<sup>***</sup>|0.0496<sup>***</sup>|
||(0.00527)|(0.00660)|(0.00672)|(0.00959)|
|FuelReductionxtreatment16|<br>0.0284<sup>***</sup>|<br>0.0191<sup>***</sup>|<br>0.0569<sup>***</sup>|<br>0.0557<sup>***</sup>|
||(0.00572)|(0.00707)|(0.00615)|(0.00824)|
|SmokeReductionxtreatment16|<br>0.0236<sup>***</sup>|<br>0.0180|<br>-0.0466<sup>***</sup>|<br>0.0561<sup>***</sup>|
||(0.00823)|(0.0112)|(0.0124)|(0.0137)|
|Costxtreatment16|<br>0.000780|<br>-0.000938|<br>-0.00569<sup>***</sup>|<br>0.00469|
||(0.00137)|(0.00169)|(0.00202)|(0.00311)|
|_2016 Control Group relative to 201_|<br>_3 Baseline_||||
|ASCxcontrol16|15.73|12.91|0.286|0.974|
||(1260.4)|(465.9)|(979.0)|(517.3)|
|StoveDurabilityxcontrol16|0.0549<sup>*</sup>|0.0794<sup>**</sup>|0.124<sup>***</sup>|0.167<sup>***</sup>|
||(0.0293)|(0.0335)|(0.0328)|(0.0421)|
|TimeReductionxcontrol16|0.00653|0.000729|-0.0453<sup>***</sup>|0.0353<sup>***</sup>|
||(0.00714)|(0.00651)|(0.0104)|(0.00993)|
|FuelReductionxcontrol16|<br>0.0311<sup>***</sup>|<br>0.0272<sup>***</sup>|<br>0.0510<sup>***</sup>|<br>0.0506<sup>***</sup>|
||(0.00846)|(0.00825)|(0.00885)|(0.00949)|
|SmokeReductionxcontrol16|<br>0.0538<sup>***</sup>|<br>0.0373<sup>***</sup>|<br>-0.0912<sup>***</sup>|<br>-0.0863<sup>***</sup>|
||(0.0159)|(0.0142)|(0.0181)|(0.0180)|
|Costxcontrol16|<br>0.00186|<br>0.000916|<br>0.00837<sup>***</sup>|<br>0.00113|
||(0.00206)|(0.00190)|(0.00236)|(0.00469)|
|Observations|17712|15318|||
|Log lik.|-3424.0|-2971.0|||
|Chi-squared|423.2|305.8|||
||Standard errors in parenthes|es<sup>*</sup> _p_< 0.10,<sup>**</sup> _p_< 0.0|5,<sup>***</sup> _p_< 0.01||

10
Table 3: Diff-in-Diff Results – WTP Estimates

|(1)|(2)|
|---|---|
|MMNL|MMNL|
|Treatment-All|Treatment-Use|
|_2013 Baseline Values_||
|ASC<br>7742.7<sup>***</sup>|5034.6<sup>***</sup>|
|(2027.4)|(1355.7)|
|StoveDurability<br>17.93<sup>***</sup>|17.48<sup>***</sup>|
|(2.717)|(2.745)|
|TimeReduction<br>4.651<sup>***</sup>|4.776<sup>***</sup>|
|(1.198)|(1.232)|
|FuelReduction<br>6.205<sup>***</sup>|6.348<sup>***</sup>|
|(1.595)|(1.648)|
|SmokeReduction<br>4.284<sup>***</sup>|4.673<sup>***</sup>|
|(1.582)|(1.659)|
|_2016 Treatment Group (total)_||
|ASCxtreatment16<br>9198.2<sup>*</sup>|12117.2|
|(4738.2)|(284992118.2)|
|StoveDurabilityxtreatment16<br>62.36<sup>**</sup>|45.66<sup>***</sup>|
|(27.32)|(13.81)|
|TimeReductionxtreatment16<br>10.31<sup>*</sup>|7.040<sup>**</sup>|
|(5.605)|(2.994)|
|<br>FuelReductionxtreatment16<br>19.22<sup>*</sup>|<br>9.331<sup>**</sup>|
|(10.26)|(3.993)|
|SmokeReductionxtreatment16<br>14.83<sup>*</sup>|7.759<sup>*</sup>|
|(8.863)|(4.273)|
|_2016 Control Group (total)_||
|ASCxcontrol<br>28254.2|11866.4|
|(858440.3)|(181816.1)|
|StoveDurabilityxcontrol16<br>78.01|54.67<sup>*</sup>|
|(95.03)|(32.83)|
|<br>TimeReductionxcontrol16<br>14.98|<br>6.765|
|(21.49)|(5.585)|
|<br>FuelReductionxcontrol16<br>35.19|<br>19.24|
|(49.78)|(14.62)|
|<br>SmokeReductionxcontrol16<br>46.31|<br>20.90|
|(66.35)|(17.07)|
|Observations<br>17712|15318|
|Standard errors in parentheses||
|* _p_< 0.10,** _p_< 0.05,*** _p_< 0.01||

Note: WTP values are presented as totals for each group (after accounting for interactions) 11
Figure 2: Marginal WTP Estimates <!-- Start of picture text --> WTP - Full Treatment<br>300<br>200<br>100<br>0<br>-100<br>StoveDurability TimeReduction FuelReduction SmokeReduction StoveDurabilityxtreatment16 TimeReductionxtreatment16 FuelReductionxtreatment16 SmokeReductionxtreatment16 StoveDurabilityxcontrol16 TimeReductionxcontrol16 FuelReductionxcontrol16 SmokeReductionxcontrol16<br><!-- End of picture text --> Marginal WTP estimates for all treatment households <!-- Start of picture text --> WTP - Full Treatment<br>100<br>80<br>60<br>40<br>20<br>0<br>StoveDurability TimeReduction FuelReduction SmokeReduction StoveDurabilityxtreatment16 TimeReductionxtreatment16 FuelReductionxtreatment16 SmokeReductionxtreatment16<br><!-- End of picture text --> Marginal WTP estimates for all treatment households – displayed without control households 12
Figure 3: Marginal WTP estimates for treatment households with long-term use <!-- Start of picture text --> WTP - Long Term Use Treatment<br>Marginal WTP estimates for treatment households with long-term use<br>WTP - Long Term Use Treatment<br>100<br>80<br>60<br>40<br>20<br>0<br>StoveDurability TimeReduction FuelReduction SmokeReduction StoveDurabilityxuse16 TimeReductionxuse16 FuelReductionxuse16 SmokeReductionxuse16 StoveDurabilityxcontrol16 TimeReductionxcontrol16 FuelReductionxcontrol16 SmokeReductionxcontrol16<br>80<br>60<br>40<br>20<br>0<br>StoveDurability TimeReduction FuelReduction SmokeReduction StoveDurabilityxuse16 TimeReductionxuse16 FuelReductionxuse16 SmokeReductionxuse16<br><!-- End of picture text --> Marginal WTP estimates for long-term use households – displayed without control households 13
Table 4: Average WTP

||Av<br>2013|erage WTP<br>2016|
|---|---|---|
|Stove Durability (1 year)|17 birr|54 birr ($2)|
|Time Reduction (10%)|47 birr|86 birr ($3)|
|Fuel Reduction (10%)|62 birr|142 birr ($5)|
|Smoke Reduction(10%)|45 birr|112 birr($4)|

Note: The average is calculated between all treatment households and long-term use treatment households. The conversation to $ (USD) is approximate (1 USD = 27.52 birr May 2018) The results support the view that the WTP estimates obtained from choice experiment surveys (for the control group) are stable, at least over a three-year period, and can therefore be used for costbenefit analysis and welfare calculations over time. The results also show that respondents in treatment groups, especially the respondents who continue to use the stoves long-term, are willing to pay significantly more for the individual benefits of the stoves than in 2013. Next, we discuss the policy implications of this in the conclusion.

# **6. Conclusions and Policy Implications**

There is a growing effort in the non-market valuation literature to better understand the stability and evolution of preferences over time. A small number of studies (Bliem et al. 2012, Brouwer and Bateman 2005, Mørkbak and Olsen 2014, Price et al. 2016) have used the test-retest method to analyze stability of preferences but have found mixed results. A combined repeated choice experiment and an RCT on stove adoption provide a unique opportunity to analyze both the stability and the evolution of preferences.

Respondents were given the exact same choice question in 2013 and 2016 and the treatment group in the RCT received improved firewood stoves, whereas the control group continued to use traditional stoves. This setting allows us to use a difference-in-difference framework to analyze the results from the choice experiment, the first instance of a difference-in-difference choice experiment analysis that we are aware of. Further, we exploit the heterogeneity in use in the treatment group to analyze the evolution of preferences in response to the effectiveness of the stoves.

We conduct a national survey in Ethiopia starting with 504 households in 36 communities in 2013, and we are able to conduct the study with 486 of the same households in 2016, giving an extremely low attrition rate of just 4%.

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Results show that WTP for stove characteristics for the control groups are stable over the study period. As noted in the introduction, this was the first test-retest choice experiment study conducted in a developing country setting. This result supports the use of values obtained from nonmarket studies over multiple years.

The results also show that the WTP for stove attributes for the treatment group increases over time and this is consistent for all stove attributes. We analyze the robustness of this result by focusing on respondents in the treatment group that are long-term stove users. It is important to note that about 1/3 of the treatment group has stopped using the stoves. We are unable to analyze this subgroup given the complexity of the estimation process and model convergence issues. Understanding the preferences of the households that have stopped using the stoves could be very important.

The results show that fuelwood reduction is the most highly valued attribute of the stoves and WTP for fuelwood reduction more than doubles between 2013 and 2016. Overall, the evidence from this study highlights that the Mirt stove, a technology developed by the Federal Government of Ethiopia in conjunction with donors, is well liked by long-term users and their overall valuation of the benefits of the improved stoves increases with long-term use of the Mirt stove.

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