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Evaluating the Economic Impact of Agricultural Policy Interventions in the Thai Jasmine Rice Markets

カムセ, キアッティザク 東京大学 DOI:10.15083/0002006898

2023.03.24

概要





















キアッティザク カムセ

カムシ キアッティザク君の博士課程学位論文“Evaluating the Economic Impacts of
Agricultural Policy Interventions in the Thai Jasmine Rice Markets”
(タイのジャスミン米市場における農業政策介入の経済的影響評価)は、タイ国(以下、
タイ)をフィールドに、様々な農業支援施策の効果を独創性にあふれた手法を用いて、定
量的かつ頑健に評価し、その政策的インプリケーションを考察したものである。
同論文の第一章は、問題意識と背景である。タイのコメ生産の実情のほか、開発途上国
における零細農家の低所得について、生産性の低さと農家庭先価格の低さという課題を述
べている。また、その根幹となる課題として、情報の非対称性や取引費用により農産物市
場が完全競争ではないことを指摘している。具体的には、中間業者が売り手の市場支配力
を行使し、多数の零細農家は低い農産物価格に直面した結果、低所得に陥っていると述べ
る。さらに農家は、農産物を売却する時期についても、信用や流動性の制約から最適化で
きず、農業収入の低下を招いていると指摘する。これまで、不完全競争分野において、中
進国での研究は非常に限られていた。また、生産者側へのドラスティックな政策的介入が
市場支配力に与える影響の評価も必ずしも十分でなかった。同論文は、その定量的な評価
を行った社会的・学術的意義が大きなものである。
第二章は、タイの籾米担保融資制度(paddy pledging program: PPP)を不完全競争モデル
を用いて定量的に評価したものである。タイのジャスミン米市場では、政権交代と連動し、
籾米を担保に農家に融資を行うという形での手厚い価格支援制度がたびたび行われてきた。
これまでのその制度評価は試みられてきたが、社会厚生に与える精緻な影響評価のために
は、既にタイのジャスミン米市場で確認されている不完全競争市場を考慮に入れた経済モ
デルを用いて分析しなければならない。そこで、本章は、NEIO のフレームワークに基づき、
ジャスミン米市場の経済モデルを新規に開発し、2001 年から 2015 年のパネルデータを用
いて同時推定を行った。買い手のパワーパラメターを推定した結果、強い買い手の市場支
配力を確認した。さらに、推定結果に基づくタイの籾米担保融資制度の評価の結果、当該
制度は市場の効率性の面で最善とはいえないが、所得の再分配機能を果たし、かつ、不完
全競争市場というタイのジャスミン米市場において、制度の実施が社会余剰全体を引き上
げうることを示した。この結果は、価格支援制度は社会余剰を低下させるという、完全競

争市場を前提としたこれまでの経済学における一般的理解と異なり、極めて意義の大きな
ファインディングであるといえる。
第三章は、タイのジャスミン米の共同販売を行う協同組合の地域への波及効果に関する
定量的評価である。これまで、協同組合へ参加した農家の販売価格が増大することは実証
的に確認されていたものの、企業の市場支配力低下によって、直接協同組合に参加してい
ない農家(その周辺に住む農家)の販売価格も増大することは理論的には指摘されていた
ものの実証的に検証されてこなかった。そこで同章は、家庭で話される言語という、コメ
生産に直接関係しないが、居住する地域と関係する操作変数を用いて、協同組合の効果を
評価した。世帯レベルのデータによる分析の結果、協同組合が作られた地域の農家は、協
同組合がない地域の農家よりも、民間仲介業者から 10.9%高い価格を受け取っていること
を明らかにした。この結果は、協同組合が地域に波及効果を持つことを示し、組合の参加
者のみをサンプリングしてその効果を検証することは、協同組合が社会福祉に与える影響
の過小評価に繋がることを示唆する。本章の結論は、農産物のバリューチェーンにおける
協同組合の意義と役割の議論に、大きな影響を与えるものと評価できる。
第四章は、タイのジャスミン米農業生産において、収穫時の信用制約を緩和する大規模
な農場貯蔵という介入が現地市場価格に与える影響の評価である。政策的介入の実施はラ
ンダムではないために、本研究では、農場貯蔵量の変数の差分に変換し、4 年と 5 年のラグ
のデータを用いることで操作変数とし、統計的な対処を実現している。本章では、タイの
19 県からの 18 年間のパネルデータを用いた実証的分析の結果、2 万トンの農場貯蔵による
介入で供給量が 6.2%減少したことで、コメ市場価格が 11 月に 1.3%、4 月に 1.2%、増大し
たことを明らかにした。またその一方、政策的介入による供給量の変化は、季節間でのコ
メ価格を安定させる効果は無かったことも示されている。農場貯蔵という介入は、大規模
に実施される場合、収穫時の過剰供給による農産物の庭先価格の低下を防ぐ効果的な手立
ての一つになりうるという興味深い結果が、実証的に示された研究といえる。
第五章は、総括である。本研究は農家の低所得問題の解決のための政策介入が、農産物
市場の機能に及ぼす影響の評価を目的としており、特に、価格支援政策、協同組合、農場
での貯蔵の支援という三つの政策を評価した。これまで社会厚生を低下させると指摘され
ていた価格支援政策が厚生を増大させる可能性を示し、協同組合のスピルオーバーを関連
研究で初めて示すなど、いずれも画期的な実証分析結果が示されていると、総合的に評価
できるものである。
以上の研究内容は、十分な学術上の新規性、及び、妥当性・有用性が確認できる。本研
究は、タイだけでなく、開発途上国・中進国に広く適用可能な、もしくは、適用が実際に
観察されている政策の、新たな視点での経済評価を実現しており、相応の社会的意義を有
すものであると判断される。研究成果は、農学と社会の進歩に対し少なくない寄与を与え
るものであり、審査委員一同は本論文が博士(農学)の学位論文として価値あるものと認
めた。

見本




















本郷 太郎

(※履歴書の記載と同じにしてください。)
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審査委員一同は本論文が博士(農学)の学位論文として価値あるものと認めた。

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Appendix

Table K1: The estimation of the effects of on-farm storage intervention using various estimation

methods

Coefficient on ∆on-farm storage quantity under the intervention (∆𝑂 𝑆𝑖𝑦 )

Estimation methods

2SLS

(1)

Dependent variable

Rice Price level (observations = 247)

Panel A: harvesting period

∆Log (𝑃𝑖 0𝑦 )

-0.0004

[0.0050]

∆Log (𝑃𝑖 𝑦 )

0.0131**

[0.0055]

∆Log (𝑃𝑖 2𝑦 )

0.0114*

[0.0066]

Panel B: Non-harvesting period

∆Log (𝑃𝑖 𝑦 )

0.0067

[0.0051]

∆Log (𝑃𝑖2𝑦 )

0.0085***

[0.0031]

∆Log (𝑃𝑖3𝑦 )

0.0054*

[0.0028]

∆Log (𝑃𝑖4𝑦 )

0.0116***

[0.0032]

∆Log (𝑃𝑖5𝑦 )

0.0086***

[0.0022]

∆Log (𝑃𝑖6𝑦 )

0.0136***

[0.0047]

∆Log (𝑃𝑖7𝑦 )

0.0164***

[0.0058]

∆Log (𝑃𝑖8𝑦 )

0.0115***

[0.0039]

∆Log (𝑃𝑖9𝑦 )

0.0083**

[0.0034]

Rice price volatility (observations = 247)

∆𝑃𝑣𝑜𝑙𝑖𝑦

-0.0024

[0.0021]

2-Step GMM

LIML

Fuller (4)-LIML

(2)

(3)

(4)

-0.0005

[0.0050]

0.0140**

[0.0055]

0.0142***

[0.0038]

0.0006

[0.0069]

0.0138**

[0.0058]

0.0116*

[0.0067]

-0.0004

[0.0050]

0.0109**

[0.0046]

0.0098*

[0.0051]

0.0093**

[0.0045]

0.0080***

[0.0025]

0.0036*

[0.0020]

0.0105***

[0.0031]

0.0078***

[0.0020]

0.0097**

[0.0043]

0.0147**

[0.0058]

0.0111***

[0.0039]

0.0066**

[0.0033]

0.0069

[0.0052]

0.0085***

[0.0031]

0.0055*

[0.0029]

0.0131***

[0.0037]

0.0088***

[0.0023]

0.0184**

[0.0075]

0.0243*

[0.0126]

0.0207

[0.0130]

0.0118**

[0.0052]

0.0056

[0.0040]

0.0064**

[0.0026]

0.0043**

[0.0021]

0.0095***

[0.0027]

0.0072***

[0.0018]

0.0137***

[0.0047]

0.0177***

[0.0067]

0.0138**

[0.0055]

0.0082**

[0.0034]

-0.0038**

[0.0018]

-0.0025

[0.0026]

-0.0024

[0.0019]

Note: The figures in brackets below the estimates are the robust standard errors. To save space,

controls for year fixed effects are not shown. *, **, *** indicate significance at the 0.1, 0.05,

0.01 levels, respectively.

160

Table K2: 2SLS estimates of the effects of on-farm storage intervention using the alternative

specification

Coefficient on ∆on-farm storage quantity under the intervention (∆𝑂 𝑆𝑖𝑦 )

2SLS

(1)

Over.

test

(2)

Dependent variables

Rice Price level (observations = 247)

Panel A: harvesting period

ΔLog (𝑃𝑖 0𝑦 )

-0.0013

0.052

[0.0051]

ΔLog (𝑃𝑖 𝑦 )

0.0126**

0.176

[0.0055]

ΔLog (𝑃𝑖 2𝑦 )

0.0111

0.638

[0.0068]

Panel B: Non-harvesting period

ΔLog (𝑃𝑖 𝑦 )

0.0062

0.335

[0.0052]

ΔLog (𝑃𝑖2𝑦 )

0.0085***

0.815

[0.0032]

ΔLog (𝑃𝑖3𝑦 )

0.0054*

0.319

[0.0030]

ΔLog (𝑃𝑖4𝑦 )

0.0119***

0.114

[0.0033]

ΔLog (𝑃𝑖5𝑦 )

0.0087***

0.281

[0.0022]

ΔLog (𝑃𝑖6𝑦 )

0.0144***

0.051

[0.0048]

ΔLog (𝑃𝑖7𝑦 )

0.0173***

0.029

[0.0059]

ΔLog (𝑃𝑖8𝑦 )

0.0123***

0.029

[0.0041]

ΔLog (𝑃𝑖9𝑦 )

0.0093**

0.081

[0.0036]

Rice price volatility (observations = 247)

Δ𝑃𝑣𝑜𝑙𝑖𝑦

-0.0022

0.258

[0.0022]

AR1

test

(3)

AR2

test

(4)

Adj. Rsquared

(5)

p-value

∆𝑂 𝑆𝑖𝑦

(6)

p-value

∆𝑂 𝑆𝑖𝑦

bootstrap (7)

0.004

0.020

0.897

0.803

0.829

0.021

0.063

0.897

0.022

0.093

0.014

0.003

0.898

0.100

0.430

0.029

0.021

0.900

0.229

0.486

0.011

0.591

0.924

0.007

0.062

0.002

0.454

0.954

0.068

0.040

0.000

0.162

0.960

0.000

0.025

0.001

0.005

0.969

0.000

0.026

0.000

0.003

0.941

0.003

0.074

0.002

0.011

0.908

0.004

0.025

0.000

0.133

0.899

0.003

0.022

0.000

0.512

0.858

0.011

0.071

0.002

0.075

0.900

0.333

0.492

Note: First stage F-statistic equals 30.65. The figures in brackets below the estimates are the

robust standard errors. To save space, controls for year fixed effects are not shown. *, **, ***

indicate significance at the 0.1, 0.05, 0.01 levels, respectively.

161

Table K3: 2SLS estimates of the effects of on-farm storage intervention without Surin and

Nakhonratchasima samples

Coefficient on ∆on-farm storage quantity under the intervention (∆𝑂 𝑆𝑖𝑦 )

2SLS

(1)

Over.

test

(2)

Dependent variables

Rice Price level (observations = 221)

Panel A: harvesting period

ΔLog (𝑃𝑖 0𝑦 )

0.0038

0.097

[0.0054]

ΔLog (𝑃𝑖 𝑦 )

0.0132***

0.191

[0.0045]

ΔLog (𝑃𝑖 2𝑦 )

0.0074**

0.275

[0.0033]

Panel B: Non-harvesting period

ΔLog (𝑃𝑖 𝑦 )

0.0032

0.125

[0.0025]

ΔLog (𝑃𝑖2𝑦 )

0.0079***

0.797

[0.0025]

ΔLog (𝑃𝑖3𝑦 )

0.0065***

0.158

[0.0020]

ΔLog (𝑃𝑖4𝑦 )

0.0117***

0.194

[0.0031]

ΔLog (𝑃𝑖5𝑦 )

0.0086***

0.435

[0.0021]

ΔLog (𝑃𝑖6𝑦 )

0.0101*

0.096

[0.0052]

ΔLog (𝑃𝑖7𝑦 )

0.0100

0.033

[0.0065]

ΔLog (𝑃𝑖8𝑦 )

0.0072

0.037

[0.0044]

ΔLog (𝑃𝑖9𝑦 )

0.0042

0.194

[0.0036]

Rice price volatility (observations = 221)

Δ𝑃𝑣𝑜𝑙𝑖𝑦

-0.0037*

0.692

[0.0020]

AR1

test

(3)

AR2

test

(4)

Adj. Rsquared

(5)

p-value

∆𝑂 𝑆𝑖𝑦

(6)

p-value

∆𝑂 𝑆𝑖𝑦

bootstrap (7)

0.009

0.080

0.8895

0.481

0.663

0.013

0.110

0.8972

0.003

0.045

0.007

0.006

0.9035

0.026

0.043

0.033

0.061

0.8946

0.269

0.212

0.013

0.522

0.9241

0.002

0.052

0.003

0.624

0.9542

0.001

0.037

0.001

0.084

0.9633

0.000

0.041

0.002

0.005

0.9700

0.000

0.018

0.000

0.002

0.9454

0.054

0.259

0.004

0.007

0.9190

0.126

0.141

0.001

0.203

0.9045

0.104

0.114

0.001

0.987

0.8559

0.244

0.269

0.003

0.117

0.8933

0.073

0.278

Note: First stage F-statistic equals 85.52. The figures in brackets below the estimates are the

robust standard errors.

To save space, controls for year fixed effects are not shown. *, **,

*** indicate significance at the 0.1, 0.05, 0.01 levels, respectively.

162

Chapter 5 General conclusion and avenues for further research

My doctoral research aims to deepen our understanding about the effect of policy

interventions that aim to solve farmers’ low-income problems on the functioning of agricultural

markets. Specifically, I evaluate three agricultural policy interventions in Thailand, including

price support policy, promoting farmer organizations, and supporting on-farm storage. These

interventions have been implemented over a decade in many developing countries. This

dissertation used data from several sources for empirical analysis. In chapter 2 and 4, I used

provincial-level data collected from several government agencies. In contrast, in chapter 3, I

used individual-level field survey data collected from two provinces in Northeast Thailand. In

this section, I first summarize the results from the dissertations. I then discuss implications for

policy and evaluation. Lastly, I discuss avenue for future research.

5.1 Summary of results

In chapter 2, I address two research questions. First, how much oligopsony power do

processors or intermediaries in the Thai Jasmine rice market have and exercise over farmers?

Second, what are the market and welfare effects of price support policy in the presence of

oligopsony? To answer the first question, I develop a rice market model consisting of rice

supply and demand equations based on the NEIO framework. To answer the second question,

I develop an imperfect competition model to evaluate the welfare effects of the Paddy Pledging

Program (PPP), a price support policy in Thailand. Using 15-year data, 15 provincial-level with

225 observations, I find that intermediaries in the Thai Jasmine rice market have oligopsony

power. The estimates of oligopsony power parameter (1 = highest level of oligopsony power)

range from -0.39 to 0.65. I also find that intermediaries exercise oligopsony power over farmers.

The estimated oligopsony price distortion ranges from -33% to 55%.

163

Using the above-estimated parameters to simulate the Thai Jasmine rice market under the

paddy pledging program, I find that the price support policy increases the farm gate price by

8.4% and reduces the consumer price by 6.35%. As a result, the program increases consumer

surplus and farmer surplus by $10.6 million and $38.8 million, respectively. However, I find

that the program is inefficient. It imposes a deadweight loss to society of about $34.9 million

per year. Nevertheless, the program can be efficient by setting an optimal support price where

the government does not have to buy rice from farmers. Next, I consider the income

redistribution effect of the program. The program is effective in income redistribution because

every public dollar spent on the program returns $1.10 in income redistribution. My findings

challenge generally accepted “wisdom” regarding price support policy in agricultural markets.

The perceived wisdom regarding this policy is that it benefits farmers, hurts consumers, and

always imposes a deadweight loss on society. Therefore, the government should eliminate the

price support policy. However, my findings show that the price support policy can benefit both

farmers and consumers in an imperfect competition market and can be designed to increase

social welfare.

In chapter 3, I test the hypothesis that nonparticipating farmers or farmers who sell

rice to private intermediaries in the areas where there is direct competition between marketing

cooperatives and private intermediaries (treated areas) are likely to receive a higher price than

those who sell rice in other areas (comparison areas). To test this hypothesis, I use language

spoken at home as an instrument. Using data from randomly selected 360 households from 36

villages in treated and comparison areas, I find that nonparticipating farmers in treated areas

receive 10.9% higher prices from private intermediaries than those who sell rice in comparison

areas. This finding provides support for the view that the presence of marketing cooperatives

can significantly force private intermediaries to competitively raise prices paid to farmers.

Therefore, promoting farmer organizations' role in the rice value chains can generate a spillover

164

effect or indirect effect.

In chapter 4, I address two research questions. First, does the change in local supply

caused by on-farm storage interventions affect equilibrium market prices? Second, is this

change in supply able to stabilize price inter-seasonally? To answer these questions, I use 4year and 5-year lagged on-farm storage quantity as instrumental variables. I find that an

increase in the on-farm storage quantity under the intervention by 20,000 tons, which is equal

to 20,000 tons decrease in local supply in the markets, causes the farm gate price in November,

February, March, April, and September to increase by 1.31%, 0.85%, 0.54%, 1.16%, and 0.83%,

respectively. Using these estimated values to calculate the welfare benefits, I find that

nonparticipating farmers gain considerable welfare benefits from on-farm storage intervention.

For example, the local supply change caused by the intervention increases the farm gate price

in November in Surin province by 7.46% or approximately $24.24 per ton. If nonparticipating

farmers in Surin sell all of their surplus paddy this month, the aggregate welfare benefits to

nonparticipating farmers in Surin will be $19.32 million. In contrast, I find that the increase in

on-farm storage quantity under the intervention does not significantly reduce price volatility.

Overall, chapter 4 shows that allowing farmers to store grains by offering them the harvesttime cash loan can affect the equilibrium market price. Hence, supporting on-farm storage can

increase farm gate prices.

165

5.2 Implications for policy and evaluation

My dissertation provides 3 crucial evidence and 7 policy implications for agri-food

policy debates regarding the welfare effect of price support policy in the presence of market

power, the role of farmer organizations in agricultural development and agricultural markets,

and the welfare implications of on-farm storage interventions when delivered on a massive

scale.

The findings in chapter 2 point out that the policy prescription to deregulate

agricultural markets in developing countries must be undertaken with caution. In an agricultural

market with oligopsony power, government policies can be warranted not only to mitigate

market distortion but also to protect small farmers and consumers from the adverse effects of

market power. In other words, my findings have highlighted the need for market interventions

when the markets function poorly due to a low competition level. In particular, when there is a

market failure, a price support policy can be designed to improve the market's efficiency and

thereby increase farmers’ income and lower consumer prices.

The finding in chapter 3 shows that strengthening the role of farmer organizations in

agricultural markets can benefit not only members but also non-members. Four implications

emerged from this finding. First, evaluating the inclusiveness of marketing cooperatives toward

poor farmers should not be limited to sampling and analyzing participating farmers only.

Second, prior studies that do not control for the spillover effect of marketing cooperatives may

underestimate the benefits of marketing cooperatives. Third, the spillover effect needs to be

incorporated in the future evaluation of the marketing cooperative’s performance. Finally,

policies aiming at enhancing the role of marketing cooperatives in rice value chains should be

aware of and address the free-rider problem to ensure that social welfare is maximized

The results in chapter 4 show that supporting on-farm storage by allowing farmers to

access credit during the harvesting time can increase the local market prices. Hence, the

166

evaluation of the economic impact of on-farm storage interventions or any investments that

will improve farmers’ ability to store needs to include its market-level effect. Moreover, onfarm storage interventions, when delivered at scale, can be used by policymakers as an effective

tool to prevent the falling local farm gate prices due to excess supply at harvest.

Overall, it is possible to raise farmers’ income through existing interventions to some

degree, and the impact assessments of these interventions need to include their spillover effects

and market-level effects.

167

5.3 Avenues for further research

There are at least three avenues for further inquiry for deepening our understanding

about the effect of policy interventions that aim to solve farmers’ low-income problems on the

functioning of agricultural markets. Firstly, we need more empirical evidence on the effects of

policy intervention on consumers. In chapter 2, we show that government policies can increase

consumers' benefits by reducing oligopolistic middlemen's rent. In chapter 3, we show that

cooperative activities have the possibility to increase consumers' benefits by reducing

oligopolistic buyers' rent. Overall, policies and cooperative activities can counter oligopsony

and oligopoly, that is, they can increase farmers' prices and may decrease consumers' prices.

Hence, it is crucial to generate more evidence on the impact of policy interventions on

consumer welfare.

The second avenue for further research is to analyze policy intervention's impact in

other vertically related markets. This is because the agricultural markets are interlinked in

complex ways. Hence, the intervention in one market may affect other vertically related

markets. As an illustration, consider a simple agricultural supply chain:

[Input providers] ➔ [Farmers] ➔ [Intermediaries] ➔ [Consumers]

where farmers buy inputs such as seeds from input providers and then sell their crops to

intermediaries such as traders and processors. And then, intermediaries sell processed crops to

consumers. In this supply chain, there are three vertically related markets: the market between

input providers and farmers, the market between farmers and intermediaries, and the market

between intermediaries and consumers. Although the policy interventions that I evaluate take

place in the market between farmers and intermediaries, it can impact other vertically related

markets as well. For example, the price support policy assessed in chapter 2 may also impact

the market between input providers and farmers. Namely, the increase in the price received by

farmers caused by the price support policy may lead to the rise in land lease fee or fertilizer

168

prices. Hence, we should analyze the impact of policy intervention in both the market where

the policy is implemented and other vertically related markets.

The third avenue for future research is to investigate how technology can be used to

solve farmers’ low-income problems. In particular, the widespread adoption of mobile phones

and the internet in rural areas creates the potential for enhancing the competition in agricultural

markets. Mobile phones and the internet can be used to enhance the functioning of agricultural

markets in developing countries in several ways. First, farmers can use mobile phones to speak

to multiple intermediaries to collect price information. This price information may allow

farmers to engage in optimal trade or arbitrage. Namely, a price difference between markets

should induce farmers to reallocate their goods to the market that offers the highest price.

Second, private sectors and governments can use a mobile phone as a platform to deliver market

information to farmers through various mobile technologies such as short messaging service

(SMS). For example, a subscription SMS service can transmit market information to farmers’

phones. Third, private sectors and governments can use the internet kiosk to deliver market

information to farmers. Lastly, private sectors and governments can set up an electronic market

where intermediaries and farmers connect over an electronic network. This electronic market

is likely to increase market competition as it integrates geographically distant markets within a

common platform. By bridging information gaps and connecting buyers with sellers, mobile

phones and the internet are likely to enhance the functioning of agricultural markets in

developing countries. Therefore, we should evaluate the impact of mobile phones and the

internet on the price received by farmers.

169

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