# What is a Normal Probability Plot?

• /
• Blog
• /
• What is a Normal Probability Plot?

A normal probability plot is a graphical representation of a data set used to assess whether the data follows a normal (bell-shaped) distribution. It is similar to a quantile-quantile plot (Q-Q Plot), which plots the quantiles of the data set against the quantiles of a normal distribution with the same mean and standard deviation as the data set.

## How to Draw a Normal Probability Plot?

Even though you can use Microsoft Excel to draw it, it does not provide an easy way to draw it. The best and the most common way to draw this plot is to use software such as Minitab, Python or R.

In Minitab, you click on Graph and then select Probability Plot... to create this plot.

## How to Interpret the Results of a Normal Probability Plot?

If the data follows a normal distribution, the points on the normal probability plot will fall approximately along a straight line. Suppose the data deviates significantly from the normal distribution. In that case, the points on the plot will not follow a straight line and may show nonlinear patterns or deviations from the theoretical normal distribution. This can indicate that the data is skewed, has shorter or longer tails than expected, or exhibits other departures from normality. For example, in the above Normal Probability Plot, all points are in a straight line enclosed by an envelope with a 95% Confidence Interval. Based on the visual look of this graph, we can confirm with 95% confidence that the data follows Normal Distribution.

A more technical approach would be to look at the P-value. Since the P-Value is more than 0.05 (it is much higher), we can say with 95% confidence that the data comes from a Normal Distribution. However, if you look at the above example, the data points do not fall in a straight line, and the P-Value is less than 0.05; we can confirm that this data comes from a Non-Normal Distribution.

## Conclusion

In conclusion, a normal probability plot is a useful graphical technique for assessing whether a sample of data comes from a normally distributed population. If the data is normally distributed, the points on the plot should roughly form a straight line. Deviations from this line may indicate that the data is not normally distributed. Normal probability plots can be a helpful tool for identifying whether a sample of data meets the assumptions of specific statistical tests or models.

``` (adsbygoogle = window.adsbygoogle || []).push({}); ```
###### About the Author Quality Gurus

Customers served! 1

Quality Management Course

FREE! Subscribe to get 52 weekly lessons. Every week you get an email that explains a quality concept, provides you with the study resources, test quizzes, tips and special discounts on our other e-learning courses.

``` (adsbygoogle = window.adsbygoogle || []).push({}); ```

Similar Posts:

December 22, 2022

## Understanding Probability: A Guide for Beginners

Understanding Probability: A Guide for Beginners

October 23, 2022

## Six Sigma Training Courses – Online or Classroom

Six Sigma Training Courses – Online or Classroom

October 24, 2022

## The Best Lean Six Sigma Interview Questions: Guaranteed to Impress! (Part 1)

The Best Lean Six Sigma Interview Questions: Guaranteed to Impress! (Part 1)

December 26, 2022

## Student’s t-Distribution

Student’s t-Distribution

January 18, 2023

## Key Behavioral Indicators (KBIs)

Key Behavioral Indicators (KBIs)

December 22, 2022

## Goodness-of-Fit Test

Goodness-of-Fit Test

November 21, 2022

## Purpose of Design of Experiments

Purpose of Design of Experiments

December 26, 2021

## Seven Quality Tools – Stratification

Seven Quality Tools – Stratification

December 22, 2022

## F-Test for Equality of Two Variances

F-Test for Equality of Two Variances

32 Courses on SALE!