t-test formula

# t-test formula - you need to set a risk level (called the...

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Taken from http://www.socialresearchmethods.net/kb/stat_t.php Figure 3. Formula for the t-test. Figure 4. Formula for the Standard error of the difference between the means. The top part of the formula is easy to compute -- just find the difference between the means. The bottom part is called the standard error of the difference . To compute it, we take the variance for each group and divide it by the number of people in that group. We add these two values and then take their square root. The specific formula is given in Figure 4: Remember, that the variance is simply the square of the standard deviation . The final formula for the t-test is shown in Figure 5: Figure 5. Formula for the t-test. The t-value will be positive if the first mean is larger than the second and negative if it is smaller. Once you compute the t-value you have to look it up in a table of significance to test whether the ratio is large enough to

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say that the difference between the groups is not likely to have been a chance finding. To test the significance,
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Unformatted text preview: you need to set a risk level (called the alpha level ). In most social research, the "rule of thumb" is to set the alpha level at .05. This means that five times out of a hundred you would find a statistically significant difference between the means even if there was none (i.e., by "chance"). You also need to determine the degrees of freedom (df) for the test. In the t-test, the degrees of freedom is the sum of the persons in both groups minus 2. Given the alpha level, the df, and the t-value, you can look the t-value up in a standard table of significance (available as an appendix in the back of most statistics texts) to determine whether the t-value is large enough to be significant. If it is, you can conclude that the difference between the means for the two groups is different (even given the variability). Fortunately, statistical computer programs routinely print the significance test results and save you the trouble of looking them up in a table....
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## This note was uploaded on 05/22/2011 for the course BIO 205 taught by Professor O'neal during the Spring '08 term at SUNY Stony Brook.

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t-test formula - you need to set a risk level (called the...

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