Chapter 9

Click on the following links - please note these will open in a new window

Navigate to the following webpage, which provides an interface for conducting a two sample t-test: http://faculty.vassar.edu/lowry/tu.html. Scroll down the page until you see a box labeled Setup. Click the Independent Samples button. We will enter data from the Latin American Migration Project (http://lamp.opr.princeton.edu) on the duration of stay, in months, in the United States during respondents' first U.S. migration. The data are presented below.

Nicaragua: 4, 6, 6, 6, 12, 36, 36, 36, 36, 60, 72, 78, 96, 120, 126, 156, 162, 162, 186, 540

Guatemala: 1, 1, 12, 24, 24, 24, 36, 36, 42, 60, 78, 84, 102, 102, 102, 102,132, 144

Locate the box labeled Sample Entry. Nicaragua will be Sample A and Guatemala will be Sample B. For each sample, enter the first value - for example, 4, if we start with Nicaragua - and then hit the return button on your keyboard. Enter the next value and then hit the return button. Do this until you have entered all values for both groups. Then click the Calculate button.

Next, look at the Data Summary box to make sure the cells are filled in. Take a moment to identify each of these quantities. Some of the notation used on this website may differ from that used in Chapter Nine. For instance, the first quantity is simply the sum, for each group, of each of the values you previously entered. Once you have identified each of the quantities in this box, move down to the box with "Meana-Meanb" in the upper left hand cell. What is the value of the calculated t-statistic? And how many degrees of freedom do you have?

To determine whether the t-statistic is statistically significant, examine the cell to the right of the "two-tailed" label. We can reject the null hypothesis if the value in this cell is less than or equal to .05 (which assumes that we set our alpha level to .05 at the outset of this test).

Finally, note the last box on the page. Here, confidence intervals are calculated around the mean for the first group, the mean for the second group, and the difference between the means for the two groups. Revisit the material in Chapter Eight as needed for a refresher on confidence intervals.