Calculate the market (S&P 500) BETA with Python for any Stock

What will we cover?

In this lesson we will learn about market Beta with S&P 500 index, how to calculate it, and comparison of calculations from last lesson.

The objective of the tutorial is:

  • Understand what market Beta tells you.
  • How to calculate the market (S&P 500) Beta.
  • See how Beta is related with Linear Regression.

Step 1: What is BETA and how to interpret the value

Beta is a measure of a stock’s volatility in relation to the overall market (S&P 500). The S&P 500 index has Beta 1.

High-beta stocks are supposed to be riskier but provide higher potential return. While, low-beta stocks pose less risk but also lower returns.

Interpretation

  • Beta above 1: stock is more volatile than the market, but expects higher return.
  • Beta below 1: stock with lower volatility, and expects less return.

The formula for Beta is Covariance divided by variance.

This sound more scary than it is.

The Beta on financial pages, like Yahoo! Finance, are calculated on the monthly price.

Step 2: Get some historic stock prices with Pandas Datareader

Let’s make an example here.

import numpy as np
import pandas_datareader as pdr
import datetime as dt
import pandas as pd
from sklearn.linear_model import LinearRegression
 
tickers = ['AAPL', 'MSFT', 'TWTR', 'IBM', '^GSPC']
start = dt.datetime(2015, 12, 1)
end = dt.datetime(2021, 1, 1)
 
data = pdr.get_data_yahoo(tickers, start, end, interval="m")
 
data = data['Adj Close']
 
log_returns = np.log(data/data.shift())

Where we notice that we read data on interval=”m”, which gives the monthly data.

Step 3: Calculate the BETA

Then the Beta is calculated as follows.

cov = log_returns.cov()
var = log_returns['^GSPC'].var()
 
cov.loc['AAPL', '^GSPC']/var

For Apple, it was 1.25.

If you wonder if it is related to the Beta value from Linear Regression. Let’s check it out.

X = log_returns['^GSPC'].iloc[1:].to_numpy().reshape(-1, 1)
Y = log_returns['AAPL'].iloc[1:].to_numpy().reshape(-1, 1)
 
lin_regr = LinearRegression()
lin_regr.fit(X, Y)
 
lin_regr.coef_[0, 0]

Also giving 1.25. Hence, it is the same calculation behind it.

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Want to learn more?

This is part of a 2.5-hour full video course in 8 parts about Risk and Return.

If you are serious about learning Python for Finance check out this course.

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