# 1. Purpose

Calculate the volatility of a stock over a period of time.

# 2-1 Volatility formula

For the price of a stock, when the stock price on any day is $S_ {i}$, the rate of change from the previous day $c_ {i}$ is defined as follows.

c_{i} = \biggl({\frac{S_i}{S_{i-1}}} \biggl)


At this time, the volatility of the stock from the start date to n business days later is defined by the following formula.

vol_{n} = \sqrt{\sum_{i=1}^{n} {(c_{i}-\bar{c})}^{2}}


# 2-2 Source code example for calculating Volatility

#### test.py


#Annual volatirity(Setting trading date 252 days)A function that calculates.
def volatility(DF):
df = DF.copy()
df["daily_ret"] = DF["Close"].pct_change()
vol = df["daily_ret"].std() * np.sqrt(252)
return vol