Hi James M. Clark Professor of Psychology 204-786-9757 204-774-4134 Fax [EMAIL PROTECTED]
>>> "Mike Palij" <[EMAIL PROTECTED]> 31-May-08 7:19:05 AM >>> On Fri, 30 May 2008 23:20:35 -0700, Jim Clark wrote: >Hi >Mike you seem to be claiming that if we let all the students with >SATs < 1200 into university this would NOT produce any increase >in the variability in GPAs? Given good reason to think that these >students would score on average at a lower level than students >with SATs > 1200, I fail to see how the standard deviation for >GPAs would stay the same. Or do you believe that scores < 1200 ?on the SAT would be associated with GPAs lat the same average >level as people with SAT scores > 1200? Consider the two colleges "College S" and "College C": ... College C only selects people with a combined SAT > 1200. and its frequency distribution of GPAs looks like thefollowing: Grade____% of Student 4.00(A)____15% 3.00(B)____30% 2.00(C)____35% 1.00(D)____15% 0.00(F)____05% ... For College C, since all values/grades in the range of possible values are being used, the addition of persons with SAT <1200 cannot increase the variance of GPAs unless they come from a non-normal population of GPA scores (e.g., a U-shaped distribution would change the variance above because the U distribution would include more extreme GPA values which would inflate the variance). Indeed, since the entrie range of values of GPAs are already in use, what HAS to happen to the variance of GPAs if we add more students? JC No, if the distribution of the added students includes more gpas of 0 and 1, and fewer gpas of 3 and 4 (e.g., below), then the SD of the combined distribution will be greater. Hypothetical distribution for SAT < 1200 Grade____% of Student 4.00(A)____5% 3.00(B)____15% 2.00(C)____35% 1.00(D)____30% 0.00(F)____15% SD depends on the proportion of observations at each level, not simply on whether or not any students occur at that level. Take care Jim --- To make changes to your subscription contact: Bill Southerly ([EMAIL PROTECTED])
