Comparing 2 groups

t-test

Mario E. Bermonti-Pérez, MA, PhD

Objectives

  • Understand the basic logic behind comparing 2 groups
  • Identify the usefulness of the independent t-test
  • Identify the basic concepts of the independent t-test
  • Conduct an independent t-test
  • Interpret the results of an independent t-test

t-test

Basics

Usefulness

Usefulness

Usefulness

General form

More general

Even more general

GLM form

GLM form - examples

GLM form - examples

GLM form - examples

General process (conceptual)

Get data

Group data

Estimate means

Estimate effect (difference)

That is all folks!

Not exactly…

We need to know:

  • is the difference real?
  • size of the difference?

Conducting and interpreting

Hands on exercises

Get ready!

Study

Effect of ADHD on planning ability

Variables

  • ADHD
    • healthy
    • ADHD
  • Plannig: Standardized test

Hypothesis

  • H0: No effect
  • H1: Effect

Info needed

  • Effect size
  • Statistical significance

Viz

  • Bar plot
  • Box plot

Descriptive stats

  • basic
  • detailed

Conducting an independent t-test

Interpretation

Effect size

Unstandardized

  • Mean difference
  • 95% CI

Standardized

Indicators

  • Hedges g
  • 95% CI

Interpretation

  • small: .2 - 0.49
  • medium: 0.5 - 0.79
  • large: 0.8+

Statistical significance

Where to look

  • Equal variances not assumed
  • Significance, two sided

Interpretation

  • p-value < 0.05 = Statistical significant
  • p-value > 0.05 = not statistical significant

Assumptions

Assumptions

  • Types of variables (GLM)
  • Normal distribution
  • Homegeinity of variance (Welch’s test) - not important
  • No outliers

Closing

Recap

  • What analysis?
  • Usefulness?
  • Predictor type?
  • Type of outcome?
  • Pieces of info t-tests provide?
  • Assumptions?

Questions?