[EMAIL PROTECTED] (Petrus Nel) wrote in message news:<000201c18fe2$f73aeee0$ed9e22c4@oemcomputer>...
> I require some advice regarding the following: One set of variables is > the grades obtained by students for different high school subjects (i.e. > the symbols candidates obtained such as A, B, C, D, etc. for each > subject). The other set of variables are the scores obtained for a > college level subject (i.e. no symbols, just their percentages > ... > The grades obtained for their high school subjects were coded on the > questionnaire as follows - 1=A, 2=B, 3=C, 4=D, 5=E, 6=F. > ... > How do I proceed? Simpler answer: First, change the coding to 1=F, 2=E, 3=D, 4=C, 5=B, 6=A. In the US at least there is no 'E'; if so, the correct coding would be 1=F, 2=D, 3=C, 4=B, 5=A. If the latter coding is used, calculate the Spearman rank correlation between the grade in a given high school course and the college score. If the former coding is used, you can use either the Pearson correlation or the Spearman rank correlation; the Pearson correlation would probably be better. More complex answer: The approach above ignores the fact that within each letter grade there is variation--e.g., all students who get a 'B' are not at the same level. Further, there is censoring at the upper end and lower ends of the scale--e.g., no matter how well a person does, the highest grade they can get is an 'A'. The polyserial correlation can account for this. The polyserial correlation estimates what the correlation of grade and score would be if grades were measured on a continuous scale. An assumption is that there is a bivariate normal distribution between (1) the continuous latent variable of which grade is a manifest representation and (2) the percentage score. The polyserial correlation is related to the polychoric correlation. For information about the polychoric correlation, see: http://ourworld.compuserve.com/homepages/jsuebersax/tetra.htm Drasgow F. Polychoric and polyserial correlations. In Kotz L, Johnson NL (Eds.), Encyclopedia of statistical sciences. Vol. 7 (pp. 69-74). New York: Wiley, 1988. I don't know if SPSS will calculate the polyserial correlation--the last I heard it did not. If not, the polyserial correlation can be calculated with the program PRELIS, which is distributed with LISREL. Many universities have copies of LISREL/PRELIS. If you are interested in comparing to see which high school classes best predict college scores, then, as a practical matter, I would expect you would draw the same conclusions regardless of whether you used the Pearson, the Spearman, or the polyserial correlation coefficients. Good luck! -------------------------------------------------------------------------------- John Uebersax, PhD (805) 384-7688 Thousand Oaks, California (805) 383-1726 (fax) email: [EMAIL PROTECTED] Agreement Stats: http://ourworld.compuserve.com/homepages/jsuebersax/agree.htm Latent Structure: http://ourworld.compuserve.com/homepages/jsuebersax Existential Psych: http://members.aol.com/spiritualpsych Diet & Fitness: http://members.aol.com/WeightControl101 -------------------------------------------------------------------------------- ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =================================================================
