1. Aim: How do we find confidence interval using SPSS? SPSS Assignment 3 due Thursday

2. Concept of Confidence Interval A confidence interval is an estimate that reflects the uncertainty inherent in random sampling.

3. Computing Confidence Interval with SPSS Analyze  Descriptive Statistics  Explore Always put a variable in the dependent list Click on Display statistics radio button (lower left corner) – It will output a Case Processing Summary followed by a long Descriptive section – Sometimes the layout of the descriptive section makes it difficult to compare the confidence interval bounds for our samples It can be fixed by pivoting the table!

4. Pivoting Descriptive Table Pivoting the descriptive table will allow you to compare confidence bounds easily – In the output: Pivot  Pivoting Trays In the columns should be: stat type and statistics In rows should be dependent variable – To move sections: click and drag the Statistics pivot icon from the Row tray to the Column tray just with Stat Type to swap the columns and rows in the table. – Close the pivot tray and the pivot table windows when completed

5. Confidence Levels SPSS uses By default, it uses 95% confidence intervals – This means: “In 95% interval estimation, about 5% (1 in 20) of all possible intervals don’t include μ. Therefore you should have approximately 95 ‘good’ intervals” – When we refer to a 95% confidence interval we are saying the 95% of all possible random samples from a population would lead to an interval containing μ. – When we conduct a study, we typically have a single sample and we don’t know if it is one of the ‘lucky 95%’ or the ‘unlucky 5%.”

6. Effect of Confidence Interval An important element of a confidence interval is the confidence coefficient (it reflects our degree of certainty about the estimate) SPSS sets confidence intervals at 95% but we can change that value as needed Generally, these coefficients are conventionally set as levels of 90%, 95%, 98%, or 99%.

7. How to change confidence intervals: Analyze  Descriptive Statistics  Explore… – In the dialogue box click on Statistics and change the level of confidence to what you need – The rest of the procedure is the same as any other confidence interval procedure Need a dependent list Click on Display

8. What does confidence interval tell you? It tells you that you are x% sure that the mean of your data falls between the confidence intervals (boundaries) – Need to check you mean with the confidence intervals… Does your mean fall within those intervals or not?

9. SPSS Assignment 3 Work on SPSS assignment