Bollinger Band code needs some minor rework #3

Inside Bollinger Bands Tutorial.py, there's some minor rework required.

item[’30 Day STD’] = item[‘Adj Close’].rolling(window=20).std()

should become

item[’30 Day STD’] = item[‘Adj Close’].rolling(window=20).std(ddof=0)
(this is because the standard deviation is being calculated against a population, not a sample)

Hope that is of assistance.

Cheers,
Richard.