# Frequency Distributions and Measures of Central Tendency

Describe frequency distributions and measures of central tendency and dispersion

## Frequency Distributions

Frequency distributions are a method of tabulating or graphically displaying a number of observations.

### The Normal Distribution

The normal distribution is a **gaussian distribution**, where the majority of values cluster around the mean, and whilst more extreme values become progressively less frequent.

The normal distribution is common in medicine for two reasons.

- Much of the variation in biology follows a normal distribution
- When multiple random samples are taken from a population, the
**mean of these samples follows a normal distribution, even if the characteristic being measured is not normally distributed**

This is known as the**central limit theorem**.- It is useful because many statistical tests are only valid when the data follow a normal distribution

The formula for the normal distribution is given by:

From this, it can bet seen the two variables which will determine the shape of the normal distribution are:

- μ (mu): The mean
- σ (sigma): The standard deviation

#### The Standard Normal Distribution

The **standard normal distribution** is a normal distribution with a mean of 0 and a standard deviation of 1. The equation for the standard normal distribution is much simpler, which is why it is used.

Any normal distribution can be transformed to fit a standard normal distribution using a **z transformation**:

The value of z then gives a **standardised score**, i.e. the number of standard deviations form the mean in a standardised curve. This can then be used to determine probability.

### Binomial distribution

Where observations belong to one of two mutually exclusive categories, i.e.:

If then

If the number of observations is very large and the probability of an event is small, a **Poisson distribution** can be used to approximate a binomial distribution.

## Measures of Central Tendency

As noted above in the **normal distribution**, results tend to cluster around a central value. Quantification of the degree of clustering can be done using **measures of central tendency**, of which there are three:

**Mode**

The most common value in the sample.**Median**

The middle value when the sample is ranked from lowest to highest.- The median is the best measure of central tendency when the data is
**skewed**

- The median is the best measure of central tendency when the data is
**Arithmetic mean**

The average, i.e:

The mean is common and reliable, though inaccurate if the distribution is**skewed**.

## Measures of Dispersion

Measures of variability describe the degree of dispersion around the central value.

### Basic Measures of Deviation

**Range**: The lowest and highest values in the sample

Highly influenced by outliers**Percentiles**: Rank observations into 100 equal parts, so that the median becomes the 50% percentile.

Better measure of spread than range.**Interquartile range**: The 25th to 75th centile

A**box-and-whisker plot**graphically demonstrates the mean, 25th centile, 75th centile, and (usually), the 10th and 90th centiles.- Outliers are represented by dots
- Occasionally the range is plotted by the whiskers, and there are no outliers plotted

### Variance and Standard Deviation

**Variance** is a better measure of variability than the above methods. Variance:

- Evaluates how far each observation is from the mean, and penalises observations more the further they lie from the mean
**Sums the squares**of each difference and divides by the number of observations i.e:- is used (instead of ) because the mean of the sample is known and therefore the last observation calculated must taken on a known quantity
- This is known as a
**degrees of freedom**, which is a mathematical restriction used when using one statistical test in order to estimate another - It is a confusing topic best illustrated with an example:
- You have been given a sample of two observations (say, ages of two individuals), and you know nothing about them
- The degrees of freedom is
**two**, since those observations can take on any value. - Alternatively, imagine you have been given the same sample, but this time I tell you that the mean age of the sample is 20
- The degrees of freedom is
**one**, since if I tell you the value of one of the observations is 30, you know that the other must be 10

Therefore, only one of the observations is free to vary - as soon as its value is known then the value of the other observation is known as well.

- Different statistical tests may result in additional losses in degrees of freedom.

- This is known as a

- is used (instead of ) because the mean of the sample is known and therefore the last observation calculated must taken on a known quantity

#### Standard Deviation

The standard deviation is the **positive square root of the variance**.

In a sample of normal distribution:

- 1 SD either side of the mean should include ~68% of results
- 2 SD either side of the mean should include ~95% of results
- 3 SD either side of the mean should include ~99.7% of results

### Standard error and Confidence Intervals

**Standard error of the mean** is:

- A measure of the precision of the estimate of the mean
- Calculated from the standard deviation and the sample size

As the sample size grows, the SEM decreases (as the estimate becomes more precise). - Given by the formula:
- Used to calculate the
**confidence interval**

#### Confidence Interval

The **confidence interval**:

- Gives a range
**in which the true population parameter is likely to lie**

The width of the interval is related to the**standard error**, and the degree of confidence (typically 95%): - Is a function of the sample statistic (in this case the mean), rather than the actual observations
- Has several benefits over the
*p*-value:- Indicates magnitude of the difference in a meaningful way
- Indicates the precision of the estimate

The smaller the confidence interval, the more precise the estimate. - Allows statistical significance to be calculated

If the confidence interval crosses 1, then the result is insignificant.

## References

- "Normal distribution". Licensed under Attribution 3.0 Unported (CC BY 3.0) via SubSurfWiki.
- Myles PS, Gin T. Statistical methods for anaesthesia and intensive care. 1st ed. Oxford: Butterworth-Heinemann, 2001
- Course notes from "Introduction to Biostats", University of Sydney, School of Public Health, circa 2013.