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股票中的分类效应

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学术论文

Categorization Bias in the Stock Market

作者作者:Philipp Kr¨uger; 机构:日内瓦大学金融研究所

机构
  • ?Augustin Landier、David Thesmar
  • ?图卢兹经济学院、巴黎HEC商学院及CEPR
论文摘要

本文提供了金融市场中存在分类偏差的证据。一些投资者通过行业的视角看待个别公司,这种分类化思维导致股票回报的错误定价和可预测性。我们通过构建基于Hoberg和Phillips (2010a,b) 方法的相关公司篮子,测量公司官方SIC行业回报与其基本面行业回报之间的差异。

研究发现,股票在短期内与其官方行业表现出强协动性,但随后逐步向其基本面行业(HP分类)的表现回归。利用行业分类引发的错误定价进行多空策略,可以产生统计显著且经济意义显著的风险调整后超额收益。

此外,研究还表明金融分析师也受到行业分类偏差的影响:当一家公司所属的官方行业与其基本面不符时,分析师往往会对官方行业的信息赋予过高权重,从而导致可预测的预测错误。这进一步支持了分类偏差对市场行为和分析师决策的深远影响。

策略概要

该策略针对NYSE、AMEX和NASDAQ的股票,结合两种行业回报数据:

官方行业回报(基于SIC分类)。

基本面行业回报(基于Hoberg和Phillips (HP) 方法)。

HP分类通过分析公司10-K表格中的产品描述,计算产品相似度来识别行业内的实际竞争者。

每周,股票根据前一周的官方行业回报与基本面行业回报的差异(回报差异)排序为五分位组合:

Q1组合:回报差异最负的股票(官方回报明显低于基本面回报)。

Q5组合:回报差异最正的股票(官方回报明显高于基本面回报)。

策略构建一个多空组合,对Q1股票做多,对Q5股票做空,投资组合按等权重配置,每周重新平衡。

策略合理性

学术研究表明,在高频率(如每周)下,股票与其官方行业的回报表现出强协动性,但与基本面行业的协动性较弱。然而,在较低频率下,这种关系会逆转。这种现象支持有限理性假说,即某些投资者倾向于过度依赖官方行业的波动,在短期内对行业冲击反应过度,而这些错误会随着时间的推移逐步被修正。

通过利用这种短期分类偏差,策略抓住了由市场错定价引发的可预测性回报。

回测表现

年化收益21.36%
波动率12.35%
贝塔0.103
夏普比率1.05
索提诺比率0.023
胜率49%

完整 Python 代码

from AlgorithmImports import *
from typing import List, Dict
import pandas as pd
import numpy as np
# endregion
class CategorizationEffectInStocks(QCAlgorithm):
def Initialize(self) -> None:
self.SetStartDate(2002, 1, 1)
self.SetCash(100_000)
self.UniverseSettings.Leverage = 5
self.UniverseSettings.Resolution = Resolution.Daily
self.AddUniverse(self.FundamentalSelectionFunction)
self.Settings.MinimumOrderMarginPortfolioPercentage = 0.0
self.settings.daily_precise_end_time = False

self.exchange_codes: List[str] = ['NYS', 'NAS', 'ASE']
self.week_period: int = 5
self.quantile: int = 5
self.selection_flag: bool = False

self.prices: Dict[str, RollingWindow] = {}
self.firm_similarity: Dict[int, Dict[str, Dict[str, float]]] = {}
self.yearly_universes: Dict[int, List[str]] = {}
self.long_symbols: List[Symbol] = []
self.short_symbols: List[Symbol] = []
csv: str = self.Download('data.quantpedia.com/backtesting_data/equity/industries/firm_similarity.csv')
lines: List[str] = csv.split('\r\n')
for line in lines[1:]: # Skip header
    if line == '':
        continue
    line_split: List[str] = line.split(';')
    year: int = int(line_split[0].split('-')[0])
    if year not in self.firm_similarity:
        self.firm_similarity[year]: Dict = {}
    if year not in self.yearly_universes:
        self.yearly_universes[year]: List = []
    industry_comp_ticker: str = line_split[1]
    if industry_comp_ticker not in self.firm_similarity[year]:
        self.firm_similarity[year][industry_comp_ticker]: Dict = {}
    if industry_comp_ticker not in self.yearly_universes[year]:
        self.yearly_universes[year].append(industry_comp_ticker)
    company_ticker: str = line_split[2]
    score: float = float(line_split[3])
    self.firm_similarity[year][industry_comp_ticker][company_ticker]: float = score
    self.yearly_universes[year].append(company_ticker)
market: Symbol = self.AddEquity('SPY', Resolution.Daily).Symbol
self.Schedule.On(
    self.DateRules.EveryDay(market), 
    self.TimeRules.BeforeMarketClose(market), 
    self.Selection)
def OnSecuritiesChanged(self, changes: SecurityChanges) -> None:
for security in changes.AddedSecurities:
    security.SetFeeModel(CustomFeeModel())
def FundamentalSelectionFunction(self, fundamental: List[Fundamental]) -> List[Symbol]:
# Update daily prices
for f in fundamental:
    ticker: str = f.Symbol.Value
    if ticker in self.prices:
        self.prices[ticker].Add(f.Price)
if not self.selection_flag:
    return Universe.Unchanged
prev_year: int = self.Time.year - 1

if prev_year not in self.firm_similarity:
    return Universe.Unchanged

# Select universe
curr_year_universe: List[str] = self.yearly_universes[prev_year]
selected: List[Fundamental] = [f for f in fundamental if f.HasFundamentalData
                        and f.Symbol.Value in curr_year_universe
                        and f.SecurityReference.ExchangeId in self.exchange_codes
                        and not np.isnan(f.AssetClassification.MorningstarIndustryGroupCode)]

warmed_up_stocks: List[Fundamental] = []
# Warm up stock prices
for f in selected:
    symbol: Symbol = f.Symbol
    ticker: str = symbol.Value
    if ticker not in self.prices:
        self.prices[ticker] = RollingWindow[float](self.week_period)
        history: pd.DataFrame = self.History(symbol, self.week_period, Resolution.Daily)
        if history.empty:
            continue
        closes: pd.Series = history.loc[symbol].close
        for _, close in closes.items():
            self.prices[ticker].Add(close)
    if self.prices[ticker].IsReady:
        warmed_up_stocks.append(f)
csv_industries: Dict[str, Dict[str, float]] = self.firm_similarity[prev_year]
industry_code_ticker: Dict[str, str] = {}
industries_groups: Dict[str, List[Symbol]] = {}
industry_diff_by_symbol: Dict[Symbol, float] = {}
symbol_by_ticker: Dict[str, Symbol] = {}
# Create industries groups
for f in warmed_up_stocks:            
    symbol: Symbol = f.Symbol
    ticker: str = symbol.Value
    industry_group_code: str = f.AssetClassification.MorningstarIndustryGroupCode
    if ticker in csv_industries:
        # Create match of csv industry group with QC industry group
        industry_code_ticker[industry_group_code]: str = ticker
        # Create match between stock's ticker and it's symbol, because this stock can be traded 
        symbol_by_ticker[ticker] = symbol
    if industry_group_code not in industries_groups:
        industries_groups[industry_group_code]: List[Symbol] = []
    industries_groups[industry_group_code].append(f.Symbol)
# Calculate difference between industry performances
for industry_group_code, ticker in industry_code_ticker.items():
    industry_universe: List[Symbol] = industries_groups[industry_group_code]
    # Official industry performance calculation
    official_industry_perf: float = np.mean([self.Performance(self.prices[symbol.Value]) 
                                                for symbol in industry_universe])
    
    # Fundamental industry performance calculation
    tickers_similarities: Dict[str, float] = self.firm_similarity[prev_year][ticker]
    fundamental_industry_perf: float = sum([self.Performance(self.prices[stock_ticker]) * similarity_score 
                                            for stock_ticker, similarity_score in tickers_similarities.items() 
                                                if stock_ticker in self.prices and self.prices[stock_ticker].IsReady])
    total_industry_score: float = sum(list(tickers_similarities.values()))
    if fundamental_industry_perf != 0 and total_industry_score != 0:
        fundamental_industry_perf /= total_industry_score
        
        trade_symbol: Symbol = symbol_by_ticker[ticker]
        curr_industries_diff: float = official_industry_perf - fundamental_industry_perf
        industry_diff_by_symbol[trade_symbol] = fundamental_industry_perf
# Make sure there are enough stocks for selection
if len(industry_diff_by_symbol) < self.quantile:
    return Universe.Unchanged
quantile: int = int(len(industry_diff_by_symbol) / self.quantile)
sorted_by_diff: List[Symbol] = [x[0] for x in sorted(industry_diff_by_symbol.items(), key=lambda item: item[1])]
self.long_symbols = sorted_by_diff[:quantile]
self.short_symbols = sorted_by_diff[-quantile:]
return self.long_symbols + self.short_symbols

def OnData(self, slice: Slice) -> None:
# Rebalance weekly
if not self.selection_flag:
    return
self.selection_flag = False

# Trade execution
targets: List[PortfolioTarget] = []
for i, portfolio in enumerate([self.long_symbols, self.short_symbols]):
    for symbol in portfolio:
        if slice.ContainsKey(symbol) and slice[symbol] is not None:
            targets.append(PortfolioTarget(symbol, ((-1) ** i) / len(portfolio)))
self.SetHoldings(targets, True)
self.long_symbols.clear()
self.short_symbols.clear()
def Performance(self, prices_roll_window: RollingWindow) -> float:
return (prices_roll_window[0] / prices_roll_window[prices_roll_window.Count - 1]) - 1
def Selection(self) -> None:
if self.Time.weekday() == 0:
    self.selection_flag = True
class CustomFeeModel(FeeModel):
def GetOrderFee(self, parameters: OrderFeeParameters) -> OrderFee:
fee: float = parameters.Security.Price * parameters.Order.AbsoluteQuantity * 0.00005
return OrderFee(CashAmount(fee, "USD"))