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standard deviation
Definition of standard deviation
1 : a measure of the dispersion of a frequency distribution that is the square root of the arithmetic mean of the squares of the deviation of each of the class frequencies from the arithmetic mean of the frequency distribution; also : a similar quantity found by dividing by one less than the number of squares in the sum of squares instead of taking the arithmetic mean
2 : a parameter that indicates the way in which a probability function or a probability density function is centered around its mean and that is equal to the square root of the moment in which the deviation from the mean is squared
First Known Use of standard deviation
1894
Financial Definition of STANDARD DEVIATION
standard deviation
What It Is
Standard deviation is a measure of how much an investment's returns can vary from its average return. It is a measure of volatility and in turn, risk. The formula for standard deviation is:
Standard Deviation = [1/n * (r{inf}i{/inf}  r{inf}ave{/inf}){sup}2{/sup}]{sup}½{/sup}
where:
r{inf}i{/inf} = actual rate of return
r{inf}ave{/inf} = average rate of return
n = number of time periods
For mathoriented readers, standard deviation is the square root of the variance.
How It Works
Let's assume that you invest in Company XYZ stock, which has returned an average 10% per year for the last 10 years. How risky is this stock compared to, say, Company ABC stock? To answer this, let's first take a closer look at the yearbyyear returns that compose that average:
At first look, we can see that the average return for both stocks over the last 10 years was indeed 10%. But let's look in a different way at how close XYZ's returns in any given year were to the average 10%:
As you can see, only during year 9 did XYZ return the average 10%. In the other years, the return was higher or lower  sometimes much higher (as in year 7) or much lower (as in year 2). Now look at the annual returns on Company ABC stock, which also had a 10% average return for the last 10 years:
As you can see, Company ABC also averaged 10% return over 10 years but did so with far less variance than Company XYZ. Its returns are more tightly clustered around that 10% average. Thus, we can say that Company XYZ is more volatile than Company ABC stock. Standard deviation seeks to measure this volatility by calculating how "far away" the returns tend to be from the average over time.
For instance, let's calculate the standard deviation for Company XYZ stock. Using the formula above, we first subtract each year's actual return from the average return, then square those differences (that is, multiply each difference by itself):
Next, we add up column D (the total is 3,850). We divide that number by the number of time periods minus one (101=9; this is called the "nonbiased" approach and it is important to remember that some calculate standard deviation using all time periods  10 in this case rather than 9). Then we take the square root of the result. It looks like this:
Standard deviation = √(3,850/9) = √427.78 = 0.2068
Using the same process, we can calculate that the standard deviation for the less volatile Company ABC stock is a much lower 0.0129.
Why It Matters
Standard deviation is a measure of risk that an investment will not meet the expected return in a given period. The smaller an investment's standard deviation, the less volatile (and hence risky) it is. The larger the standard deviation, the more dispersed those returns are and thus the riskier the investment is.
Many technical indicators, such as Bollinger Bands, incorporate the notion of standard deviation as a way to determine whether to buy or sell a stock, but it is important to remember that standard deviation is only one of many measures of risk and should not be the last word in deciding whether a stock is "too risky" or "not risky enough."
Learn More about standard deviation

Britannica.com: Encyclopedia article about standard deviation
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