Advanced Math/SPSS grief

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Expert: - 11/14/2008


Question
Hi. My daughter (a university student herself) turned me on to this website coz I am really stressed out. I'm doing a research PhD long distance. It's a mixed-method study about the predictive validity of an English proficiency exam. I haven't had any formal instruction in SPSS and making sense of the results of tests I need to perform. I've bought 2 books on understanding statistics, and I'm fairly confident about making sense of the results of Pearson's correlations. However, I need to freeze out of the correlation three cases which have anomalies. How do I do that? Also, I have the results of a questionnaire I administered to sts. to explore factors which may positively affect the predictive validity of the exam. Now, from my readings, I understand that a multiple linear regression can be used to find such factors. But the book also says that I will need to invent 'dummy variables' for each item - and there are 35 items with four options each! Question: Couldn't I use the collated results of the questionnaire (i.e. Q #1: a=80%, b=10%, c=6%, d=4%) and compare these overall results with the proficiency exam and final exam results? And just to make this request even more ridiculous, I am committed to presenting the initial results of my statistical analyses at a professional language testing conference on November 21. I'm sorry for asking you to help me at such short notice, but I'd been depending on the statistician at my univ., but he seems to have disappeared! aargh!! Can you help me.... pleeeeez?

Answer
I would look at the three case individually and find the mean and variance for each case.

Do a comparison on each one by seeing how many standard deviations the average of that item is from the overall average.  This will tell you the confidence you have of them being equal when you look at the normal distribution.  It is usually good to use 3 st dev (over 99% confidence) to see if they are different.

If you need a table for the normal distribution, it is at
http://www.sjsu.edu/faculty/gerstman/EpiInfo/z-table.htm
It gives the chance of a variable being less than or equal to that many standard deviations.  Subtract from 1 and double to see the chance of being outside for a two sided test.

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