# Correlation and Correlation Coeifficent

Article: Estimating premorbid IQ from the demographic variables: A comparison of a regression equation vs clinical judgement, Crawford et al.

1. What is the correlation between IQ from regression equation and obtained IQ?

2, What is the correlation between IQ estimated by rater 4 and obtained IQ, and how do these compare?

3. What values can a correlation coefficient take?

4. What are my practical implications?

See the attached file.

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Please see response attached.

1. What is the correlation between IQ from regression equation and obtained IQ?

The obtained IQ is given as a mean IQ score = 102.8 (Table 3. p. 4). Thus, the authors calculated an average of the participants (N = 60) IQ scores from the WAIS-R, which equaled 102.8.

The correlation between obtained IQ and the equation-based estimate is .76 (See table 4, p.5), which was significantly higher than the correlation between obtained correlation and the clinician's estimates (ranging from .64 - .68) (see Table 4, p. 5). This means that the equation-based estimate is a more accurate estimate.

Also, the mean estimated IQ from the regression model (102.5) did not differ significantly from the mean obtained IQ (102.8) whereas the means for four of the eight clinicians differ significantly from the obtained average IQ. The means estimated IQ for four clinicians (Rater 2 = 106.8; Rater 3 = 110.3; Rater 5 = 99.3; and Rater 7 = 107) differed significantly from the obtained 1Q mean (102.8). Note how Rater 2 (106.8), Rater 3 (110.3) and Rater 7 (107) had significantly higher estimates compared to the obtained IQ= 102.8; whereas Rater 5 had a significantly lower estimate of IQ (99.3) than the average obtained IQ (102.8) (see Table 3, p.4).

2. What is the correlation between IQ estimated by rater 4 and obtained IQ, and how do these compare?

The correlation between IQ estimated by Rater 4 and obtained IQ is .63 (see Table 4, p. 5).

For the second part of this question, are you asking how the Rater 4 estimate (.63) compares to the equation-based estimate (.76)? That ...

#### Solution Summary

This solution responds to the questions related to the article: Estimating premorbid IQ from the demographic variables: A comparison of a regression equation vs clinical judgement, Crawford et al.