Because the use of the PBOI for this study differed from the original purposes of the instrument, its psychometric properties were evaluated for this sample.
The PBOI items were also factor analyzed using the principal components extraction method and an oblique rotation, a method identical to the factor methods used in the instrument's development (Campbell et al.
A third preliminary analysis was performed to examine the frequency with which the clinician respondents endorsed the PBOI items (i.
answered affirmatively) was calculated using the items within each PBOI scale corresponding to his stated birth order.
On the basis of these results, I concluded that the PBOI was a valid tool for assessing the participants' birth-order-related impressions of Paul.
Paul's stated birth order was the between-subjects factor and the four PBOI scales (i.
A significant interaction of the between- and within-subjects factors was observed, which indicated that the participants' PBOI scores depended on the birth-order condition in which they were placed, F(9, 873) = 8.
No significant differences were observed between the first and middle child groups on the PBOI Middle scale, the youngest and only child groups on the Youngest scale, and the first and only child groups on the Only scale.
Finally, the participants' theoretical orientation, gender, amount of professional experience, and proportion of career counseling cases in their caseloads were not significantly related to their PBOI scores.
In contrast to the analyses of variance just discussed, the PBOI items in this analysis were used as independent variables to predict the birth-order group to which participants were assigned originally.
A quadratic discriminant function was calculated that correctly classified 73% of the respondents into their respective birth-order conditions based on their PBOI responses.