1. Lecture 15: Independent- Samples t Test 2011, 11, 15

2. Learning Objectives When to use independent-samples t-test? Defining the distribution of sample means** How to conduct an independent-sample t- test?**

3. Independent-Samples t-Test When we want to compare two samples that representing two populations (e.g., women vs. men), and the two samples are independent, we use independent-samples t-test to exam if one population mean is greater/less/equal to the other population mean. Key terms: You must have two set of scores The two set of scores must from two independent samples

4. Example 1. Teenagers Hooked on Nicotine A researcher wants to know if girls are different from boys in the degree of addiction to nicotine once they begin experimenting with smoking. The 332 students who had ever used tobacco by the start of the study were sampled from two Illinois cities. Of them, 150 were girls, and 182 were boys. They answered a questionnaire called the Hooked on Nicotine Checklist (HONC). This is a list of ten questions such as “Have you ever tried to quit but couldn’t?” The higher the score, the more hooked on nicotine a student is judged to be.

5. Example 1. Teenagers Hooked on Nicotine Data Form Research and Null Hypotheses: HONC Score GroupnSSSD Girls1502.81931.043.6 Boys1821.61522.222.9

6. Hypothesized Population  F -  M = 0 Probability Mean Diff. = Sample mean diff. n = n F + n M = 150 + 182 = 332 Your Sample:

7. Example 1. Teenagers Hooked on Nicotine Step 1: State Hypotheses Step 2: Set Criteria Two-tailed test df = 330 t crit =  1.96 Step 3: Compute Std. Err.  = 0.05 p=.025

8. Example 1. Teenagers Hooked on Nicotine Step 4: Compute t obs Step 5: Compare t obs to t crit and make conclusion. Since t obs is greater than t crit, we reject the null hypothesis.  = 0.05 p=.025

9. Recap for Today ’ s Lecture 1. When to use independent-samples t test? 2. How to compute the mean and std. Err. For the distribution of sample means? Mean ( ) Std. Err. ( )  Step 1. Compute the sum of squares  Step 2. Compute the pooled variance  Step 3. Compute the Std. Err. 3. How to compute the t obs ?