机构投资者、高管变更与股价波动
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摘要
基于2004年—2012年的中国上市公司数据,从高管变更的视角研究了机构投资者的公司治理效应,并尝试从投资者行为的角度解析高管变更事件对公司股价波动的影响机理.实证结果表明:第一,对于那些业绩亏损的公司,机构投资者的撤离能够形成较强的外部压力,迫使当任高管离职.而对于那些业绩仍然维持盈利的公司,机构投资者的施压作用则不明显.总体上,业绩好坏始终是公司高管被迫离任的主要原因,而机构投资者则是通过"用脚投票"的方式起到了推波助澜的作用.第二,高管强制变更会被机构投资者解读为负面信号,新高管的上任并不会挽回机构投资者的持股信心.第三,噪音交易者(中小个体投资者)倾向于将亏损公司的高管变更事件解读为利好消息,其踊跃买入行为则构成了公司股价在高管强制变更后发生波动加剧现象的直接原因.
Based on China's listed company data from 2004 to 2012,this paper studies institutional investors' corporate governance effect from the perspective of top management turnover,and tries to find out the influence mechanism of top management turnover events on stock volatility from the perspective of investor behavior.The empirical results show that:1) For those companies which suffer losses,institutional investors evacuation can form a strong external pressure,forcing executives to leave.But for those companies which still remain profitable,institutional investor's pressure is not obvious.Overall,business performance has always been the main reason for top management turnover,and institutional investors in China have always played their part by "voting with their feet";2) Compulsory executive alteration is a negative signal to institutional investors,the new executive cannot restore the institutional investors 'confidence in the stock holding;3) The noise traders(small individual investors) tend to interpret executives turnover in companies suffering losses as good news,whose active buying enlarges the fluctuation of the stock price directly after the turnover happened.
Based on China's listed company data from 2004 to 2012,this paper studies institutional investors' corporate governance effect from the perspective of top management turnover,and tries to find out the influence mechanism of top management turnover events on stock volatility from the perspective of investor behavior.The empirical results show that:1) For those companies which suffer losses,institutional investors evacuation can form a strong external pressure,forcing executives to leave.But for those companies which still remain profitable,institutional investor's pressure is not obvious.Overall,business performance has always been the main reason for top management turnover,and institutional investors in China have always played their part by "voting with their feet";2) Compulsory executive alteration is a negative signal to institutional investors,the new executive cannot restore the institutional investors 'confidence in the stock holding;3) The noise traders(small individual investors) tend to interpret executives turnover in companies suffering losses as good news,whose active buying enlarges the fluctuation of the stock price directly after the turnover happened.
引文
[1]Parrino R,Sias R W,Starks L T.Voting with their feet:Institutional ownership changes around forced CEO turnover[J].Journal of Financial Economics,2003,68(1):3-46.
[2]Helwege J,Intintoli V J,Zhang A.Voting with their feet or activism?Institutional investors'impact on CEO turnover[J].Journal of Corporate Finance,2012,18(1):22-37.
[3]张小蒂,李风华.机构投资者的策略变动与上市公司治理[J].财贸经济,2001,(11):18-22.Zhang Xiaodi,Li Fenghua.The tactical shifts of international investors and the corporate governance of public companies[J].Finance&Trade Economics,2001,(11):18-22.(in Chinese)
[4]汪忠,孙耀吾,龚红.机构投资者参与公司治理研究综述[J].经济学动态,2005,(10):109-114.Wang Zhong,Sun Yaowu,Gong Hong.A review about the participation of institutional investors in corporate governance[J].Economic Perspectives,2005,(10):109-114.(in Chinese)
[5]钱露.机构投资者参与公司治理的决策研究[J].经济学动态,2011,(4):38-41.Qian Lu.A decision research about the participation of institutional investors in corporate governance[J].Economic Perspectives,2011,(4):38-41.(in Chinese)
[6]王雪荣,董威.中国上市公司机构投资者对公司绩效影响的实证分析[J].中国管理科学,2009,17(2):15-20.Wang Xuerong,Dong Wei.An empirical analysis about the impact of institutional investors of Chinese public companies on corporate performance[J].Chinese Journal of Management Science,2009,17(2):15-20.(in Chinese)
[7]张涤新,李忠海.机构投资者对其持股公司绩效的影响研究——基于机构投资者自我保护的视角[J].管理科学学报,2017,20(5):82-101.Zhang Dixin,Li Zhonghai.Effect of institutional holdings on corporate performance:From the perspective of the institutional investors'self-protection[J].Journal of Management Sciences in China,2017,20(5):82-101.(in Chinese)
[8]孔东民,林之阳.机构持股、融资约束与公司分红[J].投资研究,2017,36(1):61-79.Kong Dongmin,Lin Zhiyang.Institutional ownership,financing constraints and corporate dividends[J].Review of Investment Studies,2017,36(1):61-79.(in Chinese)
[9]薄仙慧,吴联生.国有控股与机构投资者的治理效应:盈余管理视角[J].经济研究,2009,44(2):81-91(160).Bo Xianhui,Wu Liansheng.The effect of the state-owned holding and the institutional investors:From the perspective of earnings management[J].Economic Research Journal,2009,44(2):81-91(160).(in Chinese)
[10]缪毅,管悦.制度环境与机构投资者治理——基于真实盈余管理的视角[J].证券市场导报,2014,(10):18-23.Miu Yi,Guan Yue.Institutional environment and institutional investor governance:Based on the perspective of true earnings management[J].Securities Market Herald,2014,(10):18-23.(in Chinese)
[11]韩勇,干胜道,张伊.机构投资者异质性的上市公司股利政策研究[J].统计研究,2013,30(5):71-75.Han Yong,Gan Shengdao,Zhang Yi.A study on the dividend policy of the public companies with institutional investors'heterogeneity[J].Statistical Research,2013,30(5):71-75.(in Chinese)
[12]Chang E C,Wong S M L.Governance with multiple objectives:Evidence from top executive turnover in China[J].Journal of Corporate Finance,2009,15(2):230-244.
[13]游家兴,徐盼盼,陈淑敏.政治关联、职位壕沟与高管变更——来自中国财务困境上市公司的经验证据[J].金融研究,2010,(4):128-143.You Jiaxing,Xu Panpan,Chen Shumin.Political connections,position trenches and executive changes:Empirical evidence from Chinese public companies in financial distress[J].Journal of Financial Research,2010,(4):128-143.(in Chinese)
[14]Jensen M C,Warner J B.The distribution of power among corporate managers,shareholders,and directors[J].Journal of Financial Economics,1988,20(1/2):3-24.
[15]Coffee Jr J C.The Future as History:The Prospects for Global Convergence in Corporate Governance and Its Implications[R].Columbia Law School Center for Law and Economic Studies Working Paper No.144.
[16]Agrawal A,Mandelker G N.Large shareholders and the monitoring of managers:The case of antitakeover charter amendments[J].Journal of Financial and Quantitative Analysis.1990,25(2):143-161.
[17]Woidtke T.Agents watching agents?:Evidence from pension fund ownership and firm value[J].Journal of Financial Economics,2002,63(1):99-131.
[18]王谨乐,史永东.机构投资者、代理成本与公司价值——基于随机前沿模型及门槛回归的实证分析[J].中国管理科学,2016,24(7):155-162.Wang Jinle,Shi Yongdong.Institutional investors,agency costs and corporate values:Empirical analysis based on stochastic frontier analysis and threshold models[J].Chinese Journal of Management Science,2016,24(7):155-162.(in Chinese)
[19]Guercio D D,Hawkins J.The motivation and impact of pension fund activism[J].Journal of Financial Economics,1999,52(3):293-340.
[20]Aggarwal R,Erel I,Ferreira M,et al.Does governance travel around the world?Evidence from institutional investors[J].Journal of Financial Economics,2011,100(1):154-181.
[21]Graves Samuel B.Institutional ownership and corporate R&D in the computer industry[J].Academy of Management Journal,1988,31(2):417-428.
[22]Denis D J,Denis D K.Performance changes following top management dismissals[J].The Journal of Finance,1995,50(4):1029-1057.
[23]Fee C E,Hadlock C J.Management turnover across the corporate hierarchy[J].Journal of Accounting and Economics,2004,37(1):3-38.
[24]Khanna N,Poulsen A B.Managers of financially distressed firms:Villains or scapegoats?[J].The Journal of Finance,1995,50(3):919-940.
[25]Furtado E P H,Rozeff M S.The wealth effects of company initiated management changes[J].Journal of Financial Economics,1987,18(1):147-160.
[26]Cools K,Praag C M V.The value relevance of top executive departures:Evidence from the Netherlands[J].Journal of Corporate Finance,2007,13(5):721-742.
[27]Pessarossi P,Weill L.Does CEO turnover matter in China?Evidence from the stock market[J].Journal of Economics&Business,2013,70(6):27-42.
[28]Dedman E,Lin W J.Shareholder wealth effects of CEO departures:Evidence from the UK[J].Journal of Corporate Finance,2000,8(1):81-104.
[29]Falkenstein E G.Preferences for stock characteristics as revealed by mutual fund portfolio holdings[J].The Journal of Finance,1996,51(1):111-135.
[30]Badrinath S G,Gay G D,Kale J R.Patterns of institutional investment,prudence,and the managerial“safety-net”hypothesis[J].Journal of Risk and Insurance,1989,56(4):605-629.
[31]Maug E,Naik N.Herding and delegated portfolio management:The impact of relative performance evaluation on asset allocation[J].Quarterly Journal of Finance,2011,1(2):265-292.
[32]Wermers R.Mutual fund herding and the impact on stock prices[J].The Journal of Finance,1999,54(2):581-622.
[33]史永东,王谨乐.中国机构投资者真的稳定市场了吗?[J].经济研究,2014,49(12):100-112.Shi Yongdong,Wang Jinle.Have the Chinese institutional investors really stabilized the market yet?[J].Economic Research Journal,2014,49(12):100-112.(in Chinese)
[34]胡金焱,亓彬.机构投资者与股市稳定性关系的实证研究——基于修正的系统广义矩估计分阶段动态面板数据分析[J].经济学动态,2012,(10):82-86.Hu Jinyan,Qi Bin.An empirical study on the relationship between institutional investors and stock market stability:Analysis of staged dynamic panel data based on modified generalized moment estimation[J].Economic Perspectives,2012,(10):82-86.(in Chinese)
[35]Dedman E.Executive turnover in UK firms:The impact of Cadbury[J].Accounting and Business Research,2003,33(1):33-50.
[36]Neumann R,Voetmann T.Top executive turnovers:Separating decision and control rights[J].Managerial and Decision Economics,2005,26(1):25-37.
[37]Wilson R.Informational economies of scale[J].The Bell Journal of Economics,1975,6(1):184-195.
[38]Defond M L,Hung M.Investor protection and corporate governance:Evidence from worldwide CEO turnover[J].Journal of Accounting Research,2004,42(2):269-312.
[39]赵震宇,杨之曙,白重恩.影响中国上市公司高管层变更的因素分析与实证检验[J].金融研究,2007,(8):76-89.Zhao Zhenyu,Yang Zhishu,Bai Chongen.Effects and empirical test on the factors influencing the change of senior management in Chinese listed companies[J].Finance Research,2007,(8):76-89.(in Chinese)
[40]Rosenbaum P R,Rubin D B.The central role of the propensity score in observational studies for causal effects[J].Biometrika,1983,70(1):41-55.
[41]Rosenbaum P R,Rubin D B.Constructing a control group using multivariate matched sampling methods that incorporate the propensity score[J].The American Statistician,1985,39(1):33-38.
[42]Becker S O,Ichino A.Estimation of average treatment effects based on propensity scores[J].The Stata Journal,2002,2(4):358-377.
[43]Kato T,Long C.Executive turnover and firm performance in China[J].The American Economic Review,2006,96(2):363-367.
[44]Becker S O,Ichino A.Estimation of average treatment effects based on propensity scores[J].Stata Journal,2002,2(4):358-377.
[45]刘晓星,张旭,顾笑贤,等.投资者行为如何影响股票市场流动性?——基于投资者情绪、信息认知和卖空约束的分析[J].管理科学学报,2016,19(10):87-100.Liu Xiaoxing,Zhang Xu,Gu Xiaoxian,et al.How does investor behavior affect stock market liquidity?Analysis of investor sentiment,information cognition and short-sale constraints[J].Journal of Management Sciences in China,2016,19(10):87-100.(in Chinese)
(2)他们是IBM总裁John Akers,通用汽车总裁Robert Stempel,运通公司总裁James Robinson,西屋电气总裁Paul Lego,柯达总裁Kay Whitmore和康柏电脑总裁Rod Canion.
(3)该处理方式与国内大部分文献一致,如文献[10,34]等.证券投资基金长期以来都是我国机构投资者的主力军,如2011年,我国沪市机构投资者交易额为7.8万亿元,其中证券投资基金交易额为7.61万亿元,占机构投资者总交易量的97.6%(数据取自2012年4月20日“上海第二届全球基金峰会”上海证券交易所副总经理徐明发言稿).
(4)与大多数类似研究中的处理方式一致,将发生在同一季度内的多次高管变更事件,视为一次变更事件.
(5)依据发生高管变更事件前一期的营业利润对全体样本进行分组,将前期营业利润大于零的公司样本定义为盈利公司组(模型3,模型4),将前期营业利润小于零的公司样本定义为亏损公司组(模型5,模型6).
(6)观察各分组的样本数,全体样本数共计55 969个,其中盈利公司组的样本数为47 474个,几乎占据了总样本量的85%,而亏损公司组的样本数只有8 495个.所以,全体样本的回归结果与盈利公司组的回归结果基本一致也是情理之中.
(7)PS值为连续变量,无法做到精确匹配,一般非精确匹配方法包括:最近邻匹配(nearest neighbor matching)、半径匹配(radius matching)与核匹配(kernel matching).本研究采用文献中最常用的最近邻匹配[44],其原则是为处理组中的每个样本寻找与之PS值最为接近的无处理组样本作为匹配对象(形成控制组).
[2]Helwege J,Intintoli V J,Zhang A.Voting with their feet or activism?Institutional investors'impact on CEO turnover[J].Journal of Corporate Finance,2012,18(1):22-37.
[3]张小蒂,李风华.机构投资者的策略变动与上市公司治理[J].财贸经济,2001,(11):18-22.Zhang Xiaodi,Li Fenghua.The tactical shifts of international investors and the corporate governance of public companies[J].Finance&Trade Economics,2001,(11):18-22.(in Chinese)
[4]汪忠,孙耀吾,龚红.机构投资者参与公司治理研究综述[J].经济学动态,2005,(10):109-114.Wang Zhong,Sun Yaowu,Gong Hong.A review about the participation of institutional investors in corporate governance[J].Economic Perspectives,2005,(10):109-114.(in Chinese)
[5]钱露.机构投资者参与公司治理的决策研究[J].经济学动态,2011,(4):38-41.Qian Lu.A decision research about the participation of institutional investors in corporate governance[J].Economic Perspectives,2011,(4):38-41.(in Chinese)
[6]王雪荣,董威.中国上市公司机构投资者对公司绩效影响的实证分析[J].中国管理科学,2009,17(2):15-20.Wang Xuerong,Dong Wei.An empirical analysis about the impact of institutional investors of Chinese public companies on corporate performance[J].Chinese Journal of Management Science,2009,17(2):15-20.(in Chinese)
[7]张涤新,李忠海.机构投资者对其持股公司绩效的影响研究——基于机构投资者自我保护的视角[J].管理科学学报,2017,20(5):82-101.Zhang Dixin,Li Zhonghai.Effect of institutional holdings on corporate performance:From the perspective of the institutional investors'self-protection[J].Journal of Management Sciences in China,2017,20(5):82-101.(in Chinese)
[8]孔东民,林之阳.机构持股、融资约束与公司分红[J].投资研究,2017,36(1):61-79.Kong Dongmin,Lin Zhiyang.Institutional ownership,financing constraints and corporate dividends[J].Review of Investment Studies,2017,36(1):61-79.(in Chinese)
[9]薄仙慧,吴联生.国有控股与机构投资者的治理效应:盈余管理视角[J].经济研究,2009,44(2):81-91(160).Bo Xianhui,Wu Liansheng.The effect of the state-owned holding and the institutional investors:From the perspective of earnings management[J].Economic Research Journal,2009,44(2):81-91(160).(in Chinese)
[10]缪毅,管悦.制度环境与机构投资者治理——基于真实盈余管理的视角[J].证券市场导报,2014,(10):18-23.Miu Yi,Guan Yue.Institutional environment and institutional investor governance:Based on the perspective of true earnings management[J].Securities Market Herald,2014,(10):18-23.(in Chinese)
[11]韩勇,干胜道,张伊.机构投资者异质性的上市公司股利政策研究[J].统计研究,2013,30(5):71-75.Han Yong,Gan Shengdao,Zhang Yi.A study on the dividend policy of the public companies with institutional investors'heterogeneity[J].Statistical Research,2013,30(5):71-75.(in Chinese)
[12]Chang E C,Wong S M L.Governance with multiple objectives:Evidence from top executive turnover in China[J].Journal of Corporate Finance,2009,15(2):230-244.
[13]游家兴,徐盼盼,陈淑敏.政治关联、职位壕沟与高管变更——来自中国财务困境上市公司的经验证据[J].金融研究,2010,(4):128-143.You Jiaxing,Xu Panpan,Chen Shumin.Political connections,position trenches and executive changes:Empirical evidence from Chinese public companies in financial distress[J].Journal of Financial Research,2010,(4):128-143.(in Chinese)
[14]Jensen M C,Warner J B.The distribution of power among corporate managers,shareholders,and directors[J].Journal of Financial Economics,1988,20(1/2):3-24.
[15]Coffee Jr J C.The Future as History:The Prospects for Global Convergence in Corporate Governance and Its Implications[R].Columbia Law School Center for Law and Economic Studies Working Paper No.144.
[16]Agrawal A,Mandelker G N.Large shareholders and the monitoring of managers:The case of antitakeover charter amendments[J].Journal of Financial and Quantitative Analysis.1990,25(2):143-161.
[17]Woidtke T.Agents watching agents?:Evidence from pension fund ownership and firm value[J].Journal of Financial Economics,2002,63(1):99-131.
[18]王谨乐,史永东.机构投资者、代理成本与公司价值——基于随机前沿模型及门槛回归的实证分析[J].中国管理科学,2016,24(7):155-162.Wang Jinle,Shi Yongdong.Institutional investors,agency costs and corporate values:Empirical analysis based on stochastic frontier analysis and threshold models[J].Chinese Journal of Management Science,2016,24(7):155-162.(in Chinese)
[19]Guercio D D,Hawkins J.The motivation and impact of pension fund activism[J].Journal of Financial Economics,1999,52(3):293-340.
[20]Aggarwal R,Erel I,Ferreira M,et al.Does governance travel around the world?Evidence from institutional investors[J].Journal of Financial Economics,2011,100(1):154-181.
[21]Graves Samuel B.Institutional ownership and corporate R&D in the computer industry[J].Academy of Management Journal,1988,31(2):417-428.
[22]Denis D J,Denis D K.Performance changes following top management dismissals[J].The Journal of Finance,1995,50(4):1029-1057.
[23]Fee C E,Hadlock C J.Management turnover across the corporate hierarchy[J].Journal of Accounting and Economics,2004,37(1):3-38.
[24]Khanna N,Poulsen A B.Managers of financially distressed firms:Villains or scapegoats?[J].The Journal of Finance,1995,50(3):919-940.
[25]Furtado E P H,Rozeff M S.The wealth effects of company initiated management changes[J].Journal of Financial Economics,1987,18(1):147-160.
[26]Cools K,Praag C M V.The value relevance of top executive departures:Evidence from the Netherlands[J].Journal of Corporate Finance,2007,13(5):721-742.
[27]Pessarossi P,Weill L.Does CEO turnover matter in China?Evidence from the stock market[J].Journal of Economics&Business,2013,70(6):27-42.
[28]Dedman E,Lin W J.Shareholder wealth effects of CEO departures:Evidence from the UK[J].Journal of Corporate Finance,2000,8(1):81-104.
[29]Falkenstein E G.Preferences for stock characteristics as revealed by mutual fund portfolio holdings[J].The Journal of Finance,1996,51(1):111-135.
[30]Badrinath S G,Gay G D,Kale J R.Patterns of institutional investment,prudence,and the managerial“safety-net”hypothesis[J].Journal of Risk and Insurance,1989,56(4):605-629.
[31]Maug E,Naik N.Herding and delegated portfolio management:The impact of relative performance evaluation on asset allocation[J].Quarterly Journal of Finance,2011,1(2):265-292.
[32]Wermers R.Mutual fund herding and the impact on stock prices[J].The Journal of Finance,1999,54(2):581-622.
[33]史永东,王谨乐.中国机构投资者真的稳定市场了吗?[J].经济研究,2014,49(12):100-112.Shi Yongdong,Wang Jinle.Have the Chinese institutional investors really stabilized the market yet?[J].Economic Research Journal,2014,49(12):100-112.(in Chinese)
[34]胡金焱,亓彬.机构投资者与股市稳定性关系的实证研究——基于修正的系统广义矩估计分阶段动态面板数据分析[J].经济学动态,2012,(10):82-86.Hu Jinyan,Qi Bin.An empirical study on the relationship between institutional investors and stock market stability:Analysis of staged dynamic panel data based on modified generalized moment estimation[J].Economic Perspectives,2012,(10):82-86.(in Chinese)
[35]Dedman E.Executive turnover in UK firms:The impact of Cadbury[J].Accounting and Business Research,2003,33(1):33-50.
[36]Neumann R,Voetmann T.Top executive turnovers:Separating decision and control rights[J].Managerial and Decision Economics,2005,26(1):25-37.
[37]Wilson R.Informational economies of scale[J].The Bell Journal of Economics,1975,6(1):184-195.
[38]Defond M L,Hung M.Investor protection and corporate governance:Evidence from worldwide CEO turnover[J].Journal of Accounting Research,2004,42(2):269-312.
[39]赵震宇,杨之曙,白重恩.影响中国上市公司高管层变更的因素分析与实证检验[J].金融研究,2007,(8):76-89.Zhao Zhenyu,Yang Zhishu,Bai Chongen.Effects and empirical test on the factors influencing the change of senior management in Chinese listed companies[J].Finance Research,2007,(8):76-89.(in Chinese)
[40]Rosenbaum P R,Rubin D B.The central role of the propensity score in observational studies for causal effects[J].Biometrika,1983,70(1):41-55.
[41]Rosenbaum P R,Rubin D B.Constructing a control group using multivariate matched sampling methods that incorporate the propensity score[J].The American Statistician,1985,39(1):33-38.
[42]Becker S O,Ichino A.Estimation of average treatment effects based on propensity scores[J].The Stata Journal,2002,2(4):358-377.
[43]Kato T,Long C.Executive turnover and firm performance in China[J].The American Economic Review,2006,96(2):363-367.
[44]Becker S O,Ichino A.Estimation of average treatment effects based on propensity scores[J].Stata Journal,2002,2(4):358-377.
[45]刘晓星,张旭,顾笑贤,等.投资者行为如何影响股票市场流动性?——基于投资者情绪、信息认知和卖空约束的分析[J].管理科学学报,2016,19(10):87-100.Liu Xiaoxing,Zhang Xu,Gu Xiaoxian,et al.How does investor behavior affect stock market liquidity?Analysis of investor sentiment,information cognition and short-sale constraints[J].Journal of Management Sciences in China,2016,19(10):87-100.(in Chinese)
(2)他们是IBM总裁John Akers,通用汽车总裁Robert Stempel,运通公司总裁James Robinson,西屋电气总裁Paul Lego,柯达总裁Kay Whitmore和康柏电脑总裁Rod Canion.
(3)该处理方式与国内大部分文献一致,如文献[10,34]等.证券投资基金长期以来都是我国机构投资者的主力军,如2011年,我国沪市机构投资者交易额为7.8万亿元,其中证券投资基金交易额为7.61万亿元,占机构投资者总交易量的97.6%(数据取自2012年4月20日“上海第二届全球基金峰会”上海证券交易所副总经理徐明发言稿).
(4)与大多数类似研究中的处理方式一致,将发生在同一季度内的多次高管变更事件,视为一次变更事件.
(5)依据发生高管变更事件前一期的营业利润对全体样本进行分组,将前期营业利润大于零的公司样本定义为盈利公司组(模型3,模型4),将前期营业利润小于零的公司样本定义为亏损公司组(模型5,模型6).
(6)观察各分组的样本数,全体样本数共计55 969个,其中盈利公司组的样本数为47 474个,几乎占据了总样本量的85%,而亏损公司组的样本数只有8 495个.所以,全体样本的回归结果与盈利公司组的回归结果基本一致也是情理之中.
(7)PS值为连续变量,无法做到精确匹配,一般非精确匹配方法包括:最近邻匹配(nearest neighbor matching)、半径匹配(radius matching)与核匹配(kernel matching).本研究采用文献中最常用的最近邻匹配[44],其原则是为处理组中的每个样本寻找与之PS值最为接近的无处理组样本作为匹配对象(形成控制组).