Stock Return calculation problem

[deanhystad]

Intuition is good, not proof. I wrote a little program to grab the opening and closing stock prices for the entire period. From this I computed a percent change based on the period opening price and period closing price (first day and last day).

import pandas_datareader as web
import pandas as pd

portfolio = {
    "BRK-B": 0.2,
    "AMZN": 0.3,
    "AAPL": 0.1,
    "NFLX": 0.2,
    "GOOG": 0.2,
}


def get_price_info(portfolio, start, end):
    data = []
    for ticker in portfolio:
        stock = web.get_data_yahoo(ticker, start=start, end=end)
        data.append([ticker, stock["Open"][0], stock["Close"][-1]])
    return pd.DataFrame(data, columns=["Stock", "Open", "Close"])


df = get_price_info(portfolio, "2013-01-01", "2014-01-01")
df["PCT Change"] = ((df["Close"] / df["Open"]) - 1) * 100
print(df)

print("\n Percent Change", df["PCT Change"].mean())

Output:
   Stock       Open       Close  PCT Change
0  BRK-B  91.320000  118.559998   29.829170
1   AMZN  12.804000   19.939501   55.728686
2   AAPL  19.779285   20.036428    1.300062
3   NFLX  13.601429   52.595715  286.692565
4   GOOG  17.918339   27.913124   55.779642

 Percent Change 85.86602506112823