# Readers ask: When to use bonferroni correction?

## Why is Bonferroni correction used?

Purpose: The Bonferroni correction adjusts probability (p) values because of the increased risk of a type I error when making multiple statistical tests.

## Is Bonferroni correction necessary?

Classicists argue that correction for multiple testing is mandatory. Epidemiologists or rationalists argue that the Bonferroni adjustment defies common sense and increases type II errors (the chance of false negatives). “No Adjustments Are Needed for Multiple Comparisons.” Epidemiology 1(1): 43-46.

## What is a Bonferroni test used for?

The Bonferroni test is a statistical test used to reduce the instance of a false positive. In particular, Bonferroni designed an adjustment to prevent data from incorrectly appearing to be statistically significant.

## How do you use the Bonferroni method?

To perform the correction, simply divide the original alpha level (most like set to 0.05) by the number of tests being performed. The output from the equation is a Bonferroni-corrected p value which will be the new threshold that needs to be reached for a single test to be classed as significant.

## How do you find the p value for Bonferroni corrected?

To get the Bonferroni corrected/adjusted p value, divide the original α-value by the number of analyses on the dependent variable.

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## What’s wrong with Bonferroni adjustments?

The first problem is that Bonferroni adjustments are concerned with the wrong hypothesis. If one or more of the 20 P values is less than 0.00256, the universal null hypothesis is rejected. We can say that the two groups are not equal for all 20 variables, but we cannot say which, or even how many, variables differ.

## What is the difference between Tukey and Bonferroni?

For those wanting to control the Type I error rate he suggests Bonferroni or Tukey and says (p. 374): Bonferroni has more power when the number of comparisons is small, whereas Tukey is more powerful when testing large numbers of means.

## Why is Anova better than multiple t tests?

Why not compare groups with multiple ttests? Every time you conduct a ttest there is a chance that you will make a Type I error. An ANOVA controls for these errors so that the Type I error remains at 5% and you can be more confident that any statistically significant result you find is not just running lots of tests.

## What is a corrected P value?

The adjusted P value is the smallest familywise significance level at which a particular comparison will be declared statistically significant as part of the multiple comparison testing.

## How does multiple testing correction work?

Perhaps the simplest and most widely used method of multiple testing correction is the Bonferroni adjustment. If a significance threshold of α is used, but n separate tests are performed, then the Bonferroni adjustment deems a score significant only if the corresponding P-value is ≤α/n.

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## What does a post hoc test tell you?

Post hoc (“after this” in Latin) tests are used to uncover specific differences between three or more group means when an analysis of variance (ANOVA) F test is significant. Post hoc tests allow researchers to locate those specific differences and are calculated only if the omnibus F test is significant.

## Is Bonferroni too conservative?

The Bonferroni procedure ignores dependencies among the data and is therefore much too conservative if the number of tests is large. Hence, we agree with Perneger that the Bonferroni method should not be routinely used.

## Why is multiple testing a problem?

In statistics, the multiple comparisons, multiplicity or multiple testing problem occurs when one considers a set of statistical inferences simultaneously or infers a subset of parameters selected based on the observed values. The more inferences are made, the more likely erroneous inferences are to occur.