Module 3 - SLP

This SLP is largely about how we can effectively use a humble and functional program called G-Power -- an excellent piece of German freeware -- that does statistical power analysis.  It can do either a priori (before the fact) or a posteriori (after the fact) analyses; before the fact analyses help you define your needed sample size; after the fact analyses help you explain why you didn't find anything interesting once you’re all done.

Free software, like free advice, is often worth just about what you paid for it; in this case, there are some limitations.  Being developed by statistically-minded Germans, it isn’t exactly oozing with warm friendlies and lots of explanations; it’s severely functional, and assumes that you know what you’re doing.   It still has two cardinal virtues, in my opinion:  (a) it's free, and (b) it works, fast and well.  It's nice to know that there's some sweet to go with the bitter.

In this Project assignment, you'll download and install G-Power (you can take it off later if you don't want to keep it), and use it in conjunction with SPSS to do some basic power analyses for different types of tests.  Then when you get ready to do your dissertations, you'll at least know the basics of sample size analysis; it won't get you the sample, but it may save you from bad ones.

So -- on to the Assignment. Please note that there is an available "gloss" file that will walk you through the analysis if you need help beyond that made available by the PowerPoint Guide presentations linked below. Don’t hesitate to use the Gloss if you need it, but the more of the work you can do without recourse to it, the more you’ll be achieving personal mastery of this material, which is important.

SLP assignment

Click here to download G-Power, and follow the instructions as to how to set it up on your machine. The G-Power tutorial is available here ; it will walk you through one basic set of analyses.

Here is the data set from the Los Angeles Unified School District that we'll be using for this assignment.

Here is a description of the data and a codebook

And here's an SPSS version of the data

1. Begin by familiarizing yourself with the data.  Calculate appropriate descriptive statistics for all the variables (remember, some are interval and some are categorical; be sure to present the appropriate statistics in each case). Add appropriate explanations to your data clarifying what the statistics tell you.
2. Test the hypothesis that there is a difference between boys and girls in their grades in the Spring Math Course.  Is the hypothesis supported?  Run the appropriate test and give the numbers that lead you to your conclusion.
3. Use G-Power ( here is a guide to using GPower with these data ) to determine the actual power of your test given your results (means, SD's, and sample sizes; use alpha=.05).  What is the size of the effect you have observed?  How do you interpret the results of your analysis?  Use Daniel Soper's free Post-hoc Statistical Power Calculator (Multiple Regression) to check your results.
4. Suppose you were going to design a correlational study in which you couldn't assume that the effect size (r) was more than .05, but you want to achieve 80% power with alpha = .05.  Use G-Power to determine how large a total sample you would need to get such a level of power.  Test your G-Power conclusions against those derived from Daniel Soper's free A-priori Sample Size Calculator (Multiple Regression) .
5. It's been alleged that boys tend to be put into bilingual education more often than girls.  Using the variables biling2 and gender , construct a 2X2 contingency table to test this hypothesis.  Is it supported?  Using G-Power, determine the post hoc power of this test to detect a true result.  If it is not supported, use G-Power to determine how large a sample would have been required to find significance in a relationship with the effect size detected in this sample.
6. When you're done, put your findings in the form of a report using Word®.  Try to format your results so that they are aesthetically pleasing, or at least no worse looking than the average academic article.

Price: $17.33

Solution: The downloadable solution consists of 15 pages, 233 words and 20 charts.
Deliverable: Word Document