Published on
July 7, 2022
by
Shaun Turney.
Revised on
June 22, 2023.

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.

Published on
July 6, 2022
by
Shaun Turney.
Revised on
June 22, 2023.

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.

Published on
June 27, 2022
by
Shaun Turney.
Revised on
January 29, 2024.

Kurtosis is a measure of the tailedness of a distribution. Tailedness is how often outliers occur. Excess kurtosis is the tailedness of a distribution relative to a normal distribution.

Distributions with medium kurtosis (medium tails) are mesokurtic.

Distributions with low kurtosis (thin tails) are platykurtic.

Distributions with high kurtosis (fat tails) are leptokurtic.

Tails are the tapering ends on either side of a distribution. They represent the probability or frequency of values that are extremely high or low compared to the mean. In other words, tails represent how often outliers occur.

Published on
June 15, 2022
by
Shaun Turney.
Revised on
November 20, 2023.

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

Published on
June 9, 2022
by
Shaun Turney.
Revised on
June 21, 2023.

A probability distribution is a mathematical function that describes the probability of different possible values of a variable. Probability distributions are often depicted using graphs or probability tables.

Example: Probability distributionWe can describe the probability distribution of one coin flip using a probability table:

Published on
June 7, 2022
by
Shaun Turney.
Revised on
June 21, 2023.

A frequency distribution describes the number of observations for each possible value of a variable. Frequency distributions are depicted using graphs and frequency tables.

Published on
May 31, 2022
by
Shaun Turney.
Revised on
June 21, 2023.

The chi-square (Χ^{2}) distribution table is a reference table that lists chi-square critical values. A chi-square critical value is a threshold for statistical significance for certain hypothesis tests and defines confidence intervals for certain parameters.

Chi-square critical values are calculated from chi-square distributions. They’re difficult to calculate by hand, which is why most people use a reference table or statistical software instead.

Published on
May 24, 2022
by
Shaun Turney.
Revised on
June 22, 2023.

A chi-square (Χ^{2}) goodness of fit test is a type of Pearson’s chi-square test. You can use it to test whether the observed distribution of a categorical variable differs from your expectations.

The chi-square goodness of fit test tells you how well a statistical model fits a set of observations. It’s often used to analyze genetic crosses.

Published on
May 23, 2022
by
Shaun Turney.
Revised on
June 22, 2023.

A Pearson’s chi-square test is a statistical test for categorical data. It is used to determine whether your data are significantly different from what you expected. There are two types of Pearson’s chi-square tests:

The chi-square goodness of fit test is used to test whether the frequency distribution of a categorical variable is different from your expectations.