Rick Since there are only 15 points (one per year? Students take the MFT only once?) one outlier can have a large influence on the r value (and p). What's the r without the outlier? Is that point flagged as an outlier in the stat program?
And I assume the residuals fit the picture of a linear fit? (Both above and below the line across the years). IF there is a direct path between size and test score, the outlier may be very illuminative i.e. whatever the true I.V. is, there is more of it in the largest class hence causing the lowest test score. As far as alternative explanations, admissions standards could cause it. Lower standards = more weaker students? Do psych enrollments correlate with overall enrollments? Sent from my BlackBerry® wireless device from U.S. Cellular -----Original Message----- From: Rick Froman <[email protected]> Date: Wed, 11 Nov 2009 09:15:17 To: Teaching in the Psychological Sciences (TIPS)<[email protected]> Subject: [tips] Corr b/w small class sizes and academic achievement We been administering the Psych MFT as an outcomes assessment for the department since 1988 and a record has been kept of the size of the graduating class and the average score of the class each year. Currently, the correlation between size of graduating class and average MFT score stands at r(21)=-.50, p=.018, r-squared = .25. I am aware of the dangers of confusing correlation with causation (I teach it every semester) so, other than the obvious causal hypothesis that "small class sizes produce better education experiences," how many competing explanations can you think of? BTW, the size of the faculty has been three for most of those years (since 1994) so I don't think S-F ratios would make a difference. Other than one outlier year with a large group with quite a low average (which tends to strengthen the negative correlation), the scatterplot appears grouped nicely along a negative regression line. Rick Dr. Rick Froman, Chair Division of Humanities and Social Sciences Box 3055 x7295 [email protected] http://tinyurl.com/DrFroman Proverbs 14:15 "A simple man believes anything, but a prudent man gives thought to his steps." --- To make changes to your subscription contact: Bill Southerly ([email protected]) --- To make changes to your subscription contact: Bill Southerly ([email protected])
