Statistical Concepts and Market Returns

8 important questions on Statistical Concepts and Market Returns

In which two broad meanings do we use statistics?

1) Descriptive statistics (how data can be summarized to information)
2) Statistical inference (making forecasts/estimates from a smaller group that is observed, based on probability theory)

How do we define a population vs. a sample?

A populations is defined as all members of a specified group. As gathering all data is time-consuming and costly, we usually use samples. A sample is a subset of a population. Anaylysts hope that this objectively represents the population. A sample statistic is a quantity computed from or used to describe a sample.

What measurement scales do we use?

- Nominal scale: weakest, cannot be ranked only categories
- Ordinal scale: can be ranked, but the scale can't tell us anything about the difference (e.g. between 1 and 2 compared to 4 and 5).
- Interval scale: differences between a scale are equal but there is no zero point (zero celsius is not the absence of temperature)
- Ratio: strongerst version, true zero point (e.g. money).
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What is a histogram, frequency polygon and CFD (cumulative frequency distribution) ?

A histogram is a bar chart of data that have been grouped into a frequency distribution. Advantage is that we can quickly see where the most observations lie.

A polygon is a straight line between the midpoints of the intervals, in which the height is determined by the absolute frequency.

A CFD or CDF is the cumulative version of a polygon, but uses the upper interval limit instead of the midpoint. Mainly useful if a statement about at least X% is above Y. In essence the slope at any point is proportional to the number of observations in the interval.

What measures of dispersion do we use?

As Fisher Black said: the key in investment is estimating expected return. However, to completely understand an investment we need to know how it is dispersed around the mean, the variability around the central tendency. Mean return adress reward, then dispersion adresses risk.

We have multiple absolute dispersion measures (no reference point or benchmark):
- Range (which is the maximum - minimum)
- Mean absolute deviation (sum would otherwise be zero)
- Variance (average of squared deviations from the arithmatic mean)
- Standard deviation (positive square root of the variance), often interpreted as an assets risk.

How do we calculate a semivariance/semideviation?

Semivariance  - - > average squared deviation below the mean (downside risk)
Semideviation - - > positive square root of the semivariance
Calculate by:
- Sample mean
- Identify observations smaller than or equal to the mean
- Compute the sum of all these squared negative deviations
- Divide by the number of observations in the sample - 1.

We can use something else than the mean as well, then it turns into target semivariance and target semideviation.

What does Chebyshev inequality present us?

Any distribution with a finite variance, the proportion of the observations within k standard deviations of the arithmetic mean is at least 1 - 1/k2, so:
1,25  - - > 36%
1,5  - - > 56%
2 - - > 75%
2,5 - - > 84%
3 - - > 89%
4 - - > 94%

Importance is because of generality as it holds for any shape of distribution, continuous or discrete data.

Why do we use the coefficient of variation?

As it is a relative measure of dispersion to a benchmark or reference value. Especially when size differ substantially this will help us out. It is the ratio of the standard deviation over the mean value.

IMPORTANT: if we use returns we measure the amount of risk (standard deviation) per unit of mean return. Which expresses the magnitude of variation among observations relative to their average size. This enables us to compare portfolios and stocks, as it is a scale free measure!! No units of measurement.

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