# Normality and independent samples t-test

## Assess normality when comparing two independent groups on a continuous outcome

The assumption of normality is the first statistical assumption that needs to be tested when comparing two independent groups on a continuous outcome. Normality of a continuous distribution is assessed using skewness and kurtosis statistics. In order to meet the statistical assumption of normality, both skewness and kurtosis statistics should be below an absolute value of 2.0. If either a skewness or a kurtosis statistic is above an absolute value of 2.0, then the continuous distribution is assumed to be non-normal. Independent samples t-tests should not be conducted on continuous variables that violate the assumption of normality. Independent samples t-tests should only be conducted on continuous outcomes that are normally distributed.

### The steps for checking the assumption of normality for independent samples t-test in SPSS

1. Click

2. Drag the mouse pointer over the

3. Select

4. Click on the continuous outcome variable to highlight it.

5. Click on the

6. Click the

7. Deselect

8. Select the

9. Click

10. Select the

11. Click

**.**__A__nalyze2. Drag the mouse pointer over the

**D**drop-down menu.__e__scriptive Statistics3. Select

**.**__D__escriptives4. Click on the continuous outcome variable to highlight it.

5. Click on the

**arrow**to move the outcome variable into the**box.**__V__ariable(s):6. Click the

**tab.**__O__ptions7. Deselect

**Mi**and__n__imum**Ma**boxes under the__x__imum**Dispersion**section.8. Select the

**and**__K__urtosis**Ske**boxes under the__w__ness**Distribution**section.9. Click

**Continue**.10. Select the

**Save standardi**box.__z__ed values as variables11. Click

**OK**.### The steps for interpreting the SPSS output for normality and independent samples t-test

1. Under the

**skewness**and**kurtosis**columns of the**Descriptive Statistics**table, if the**Statistic is less than an absolute value of 2.0**, then researchers can assume**normality**.### Was the assumption of normality met for independent samples t-test?

## Hire A Statistician

**DO YOU NEED TO HIRE A STATISTICIAN?**

Eric Heidel, Ph.D., PStat** **will provide the following statistical consulting services for undergraduate and graduate students at $100/hour. Secure checkout is available with Stripe, Venmo, Zelle, or PayPal.

- Statistical Analysis
- Research Design
- Sample Size Calculations
- Diagnostic Testing and Epidemiological Calculations
- Survey Design and Psychometrics