## Skewness | Definition, Examples & Formula

Skewness is a measure of the asymmetry of a distribution. A distribution is asymmetrical when its left and right side are not mirror images.

A distribution can have right (or positive), left (or negative), or zero skewness. A right-skewed distribution is longer on the right side of its peak, and a left-skewed distribution is longer on the left side of its peak:

You might want to calculate the skewness of a distribution to:

• Describe the distribution of a variable alongside other descriptive statistics
• Determine if a variable is normally distributed. A normal distribution has zero skew and is an assumption of many statistical procedures.

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## Poisson Distributions | Definition, Formula & Examples

A Poisson distribution is a discrete probability distribution. It gives the probability of an event happening a certain number of times (k) within a given interval of time or space.

The Poisson distribution has only one parameter, λ (lambda), which is the mean number of events. The graph below shows examples of Poisson distributions with different values of λ.

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## Central Limit Theorem | Formula, Definition & Examples

The central limit theorem states that if you take sufficiently large samples from a population, the samples’ means will be normally distributed, even if the population isn’t normally distributed.

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## How to Find Degrees of Freedom | Definition & Formula

Degrees of freedom, often represented by v or df, is the number of independent pieces of information used to calculate a statistic. It’s calculated as the sample size minus the number of restrictions.

Degrees of freedom are normally reported in brackets beside the test statistic, alongside the results of the statistical test.

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## Null and Alternative Hypotheses | Definitions & Examples

The null and alternative hypotheses are two competing claims that researchers weigh evidence for and against using a statistical test:

• Null hypothesis (H0): There’s no effect in the population.
• Alternative hypothesis (HA): There’s an effect in the population.

The effect is usually the effect of the independent variable on the dependent variable.

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## Systematic Review | Definition, Examples & Guide

A systematic review is a type of review that uses repeatable methods to find, select, and synthesise all available evidence. It answers a clearly formulated research question and explicitly states the methods used to arrive at the answer.

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