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Lecture 15: Independent-Samples t Test

Lecture 15: Independent-Samples t Test. 2011, 11, 15. Learning Objectives. When to use independent-samples t -test? Defining the distribution of sample means** How to conduct an independent-sample t -test?**. Independent-Samples t -Test.

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Lecture 15: Independent-Samples t Test

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  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:

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

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

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

  9. Recap for Today’s Lecture • When to use independent-samples t test? • 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. • How to compute the tobs?

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