Quantitative Methods  Standard Deviation And Variance
Range and Mean Absolute Deviation
The range is the simplest measure of dispersion, the extent to which the data varies from its measure of central tendency. Dispersion or variability is a concept covered extensively in the CFA curriculum, as it emphasizes risk, or the chances that an investment will not achieve its expected outcome. If any investment has two dimensions  one describing risk, one describing reward  then we must measure and present both dimensions to gain an idea of the true nature of the investment. Mean return describes the expected reward, while the measures of dispersion describe the risk.
Range
Range is simply the highest observation minus the lowest observation. For data that is sorted, it should be easy to locate maximum/minimum values and compute the range. The appeal of range is that it is simple to interpret and easy to calculate; the drawback is that by using just two values, it can be misleading if there are extreme values that turn out to be very rare, and it may not fairly represent the entire distribution (all of the outcomes).
Mean Absolute Deviation (MAD)
MAD improves upon range as an indicator of dispersion by using all of the data. It is calculated by:
1. Taking the difference between each observed value and the mean, which is the deviation
2. Using the absolute value of each deviation, adding all deviations together
3. Dividing by n, the number of observations.
Example:
To illustrate, we take an example of six midcap mutual funds, on which the fiveyear annual returns are +10.1, +7.7%, +5.0, +12.3%, +12.2% and +10.9%.
Answer:
Range = Maximum  Minimum = (+12.3%)  (+5.0%) = 7.3%
Mean absolute deviation starts by finding the mean: (10.1% + 7.7% + 5.0% + 12.3% + 12.2% + 10.9%)/6 = 9.7%.
Each of the six observations deviate from the 9.7%; the absolute deviation ignores +/.
1^{st}: 10.1  9.7 = 0.4 3^{rd}: 5.0  9.7 = 4.7 5^{th}: 12.2  9.7 = 2.5
2^{nd}: 7.7  9.7 = 2.0 4^{th}: 12.3  9.7 = 2.6 6^{th}: 10.9  9.7 = 1.2
Next, the absolute deviations are summed and divided by 6:(0.4 + 2.0 + 4.7 + 2.6 + 2.5 + 1.2)/6 = 13.4/6 = 2.233333, or rounded, 2.2%.
Variance
Variance (Ïƒ^{2}) is a measure of dispersion that in practice can be easier to apply than mean absolute deviation because it removes +/ signs by squaring the deviations.
Returning to the example of midcap mutual funds, we had six deviations. To compute variance, we take the square of each deviation, add the terms together and divide by the number of observations.
Observation  Value  Deviation from +9.7%  Square of Deviation 
1  +10.1%  0.4  0.16 
2  +7.7%  2.0  4.0 
3  +5.0%  4.7  22.09 
4  +12.3%  2.6  6.76 
5  +12.2%  2.5  6.25 
6  +10.9%  1.2  1.44 
Variance = (0.16 + 4.0 + 22.09 + 6.76 + 6.25 + 1.44)/6 = 6.7833. Variance is not in the same units as the underlying data. In this case, it's expressed as 6.7833% squared  difficult to interpret unless you are a mathematical expert (percent squared?).
Standard Deviation
Standard deviation (Ïƒ) is the square root of the variance, or (6.7833)^{1/2} = 2.60%. Standard deviation is expressed in the same units as the data, which makes it easier to interpret. It is the most frequently used measure of dispersion.
Our calculations above were done for a population of six mutual funds. In practice, an entire population is either impossible or impractical to observe, and by using sampling techniques, we estimate the population variance and standard deviation. The sample variance formula is very similar to the population variance, with one exception: instead of dividing by n observations (where n = population size), we divide by (n  1) degrees of freedom, where n = sample size. So in our mutual fund example, if the problem was described as a sample of a larger database of midcap funds, we would compute variance using n  1, degrees of freedom.
Sample variance (s^{2}) = (0.16 + 4.0 + 22.09 + 6.76 + 6.25 + 1.44)/(6  1) = 8.14
Sample Standard Deviation (s)
Sample standard deviation is the square root of sample variance:
(8.14)^{1/2} = 2.85%.
In fact, standard deviation is so widely used because, unlike variance, it is expressed in the same units as the original data, so it is easy to interpret, and can be used on distribution graphs (e.g. the normal distribution).
Semivariance and Target Semivariance
Semivariance is a risk measure that focuses on downside risk, and is defined as the average squared deviation below the mean. Computing a semivariance starts by using only those observations below the mean, that is, any observations at or above the mean are ignored. From there, the process is similar to computing variance. If a return distribution is symmetric, semivariance is exactly half of the variance. If the distribution is negatively skewed, semivariance can be higher. The idea behind semivariance is to focus on negative outcomes.
Target semivariance is a variation of this concept, considering only those squared deviations below a certain target. For example, if a mutual fund has a mean quarterly return of +3.6%, we may wish to focus only on quarters where the outcome is 5% or lower. Target semivariance eliminates all quarters above 5%. From there, the process of computing target semivariance follows the same procedure as other variance measures.
Chebyshev's Inequality
Chebyshev's inequality states that the proportion of observations within k standard deviations of an arithmetic mean is at least 1  1/k^{2}, for all k > 1.
# of Standard Deviations from Mean (k)  Chebyshev\'s Inequality  % of Observations 
2  1  1/(2)^{2}, or 1  1/4, or 3/4  75 (.75) 
3  1  1/(3)^{2}, or 1  1/9, or  89 (.8889) 
4  1  1/(4)^{2}, or 1  1/16, or 15/16  94 (.9375) 
Given that 75% of observations fall within two standard deviations, if a distribution has an annual mean return of 10% and a standard deviation of 5%, we can state that in 75% of the years, the return will be anywhere from 0% to 20%. In 25% of the years, it will be either below 0% or above 20%. Given that there are 89% falling within three standard deviations means that in 89% of the years, the return will be within a range of 5% to +25%. Eleven percent of the time it won't.
Later we will learn that for socalled normal distributions, we expect about 95% of the observations to fall within two standard deviations. Chebyshev's inequality is more general and doesn't assume a normal distribution, that is, it applies to any shaped distribution.
Coefficient of Variation
The coefficient of variation (CV) helps the analyst interpret relative dispersion. In other words, a calculated standard deviation value is just a number. Does this number indicate high or low dispersion? The coefficient of variation helps describe standard deviation in terms of its proportion to its mean by this formula:
Formula 2.18 CV = s/X Where: s = sample standard deviation, X = sample mean 
Sharpe Ratio
The Sharpe ratio is a measure of the riskreward tradeoff of an investment security or portfolio. It starts by defining excess return, or the percentage rate of return of a security above the riskfree rate. In this view, the riskfree rate is a minimum rate that any security should earn. Higher rates are available provided one assumes higher risk.
The Sharpe ratio is calculated by dividing the ratio of excess return, to the standard deviation of return.
Formula 2.19 Sharpe ratio = [(mean return)  (riskfree return)] / standard deviation of return 
Skew And KurtosisExample: Sharpe Ratio
If an emergingmarkets fund has a historic mean return of 18.2% and a standard deviation of 12.1%, and the return on threemonth Tbills (our proxy for a riskfree rate) was 2.3%, the Sharpe ratio = (18.2)(2.3)/12.1 = 1.31. In other words, for every 1% of additional risk we accept by investing in this emerging markets fund, we are rewarded with an excess 1.31%. Part of the reason that the Sharpe ratio has become popular is that it's an easy to understand and appealing concept, for practitioners and investors.

Professionals
Expected Return, Variance And Standard Deviation Of A Portfolio
Learn these valuable ways of analyzing your portfolio. 
Investing
Standard Deviation
Learn about how standard deviation is applied to the annual rate of return of an investment to measure the its volatility. 
Mutual Funds & ETFs
Standard Deviation
Standard Deviation 
Mutual Funds & ETFs
Standard Deviation & Value At Risk
Standard Deviation & Value At Risk 
Professionals
Measuring Portfolio Risks
FINRA/NASAA Series 66: Section 5 Measuring Portfolio Risks. This section discusses different risk measures: Beta, Alpha and the Sharpe Ratio. 
Professionals
Confidence Intervals
CFA Level 1  Common Probability Distributions  Confidence Intervals 
Forex Education
Trading With Gaussian Models Of Statistics
The entire study of statistics originated from Gauss and allowed us to understand markets, prices and probabilities, among other applications. 
Fundamental Analysis
Explaining the Empirical Rule
The empirical rule provides a quick estimate of the spread of data in a normal statistical distribution. 
Professionals
Stock Risks
NASAA Series 65: Section 16 Stock Risks. In this section types of risks, quantitative analysis and risk measures associated to stock markets. 
Bonds & Fixed Income
Find The Highest Returns With The Sharpe Ratio
Learn how to follow the efficient frontier to increase your chances of successful investing.

Standard Deviation
1. A measure of the dispersion of a set of data from its mean. ... 
Semivariance
A measure of the dispersion of all observations that fall below ... 
Residual Standard Deviation
A statistical term used to describe the standard deviation of ... 
Empirical Rule
A statistical rule stating that for a normal distribution, almost ... 
Downside Deviation
A measure of downside risk that focuses on returns that fall ... 
Standard Error
The standard deviation of the sampling distribution of a statistic. ...

What is the difference between standard deviation and average deviation?
Understand the basics of standard deviation and average deviation, including how each is calculated and why standard deviation ... Read Answer >> 
How is standard deviation used to determine risk?
Understand the basics of calculation and interpretation of standard deviation and how it is used to measure risk in the investment ... Read Answer >> 
What does standard deviation measure in a portfolio?
Dig deeper into the investment uses of, and mathematical principles behind, standard deviation as a measurement of portfolio ... Read Answer >> 
What is the difference between variance and standard deviation?
Explore the differences between standard deviation and variance. Learn more about how statisticians use these two concepts. Read Answer >> 
What is the difference between standard deviation and mean?
Understand the basics of calculating and interpreting mean and standard deviation and how these mathematical fundamentals ... Read Answer >> 
What is the difference between standard deviation and variance?
Understand the difference between standard deviation and variance; learn how each is calculated and how these concepts are ... Read Answer >>