Two separate models were run, with IAWL and ORF as the outcome measures.
The ORF and IAWL initial status and slope parameters were then entered in multiple regression analysis to determine the unique variance explained (expressed as [R.sup.2]) by the IAWL and ORF slope parameters in predicting raw change scores on standardized tests, after controlling for a student's initial status on the IAWL and ORF measures.
The hierarchical coding scheme used to determine the rank ordering of words on the IAWL was evaluated using a one-parameter model based on item response theory (IRT; Bock, 1997; Embretson, 1996; Embretson & Reise, 2000).
Despite the GEE analysis demonstrating the overall ability of the IAWL to predict when words become decodable and the ability of the taught lessons to increase the percentage passing, IRT analysis showed considerable shifting in the difficulty rank ordering.
Results indicated that the IAWL accounted for unique variance on measures of timed and untimed word reading and decoding and timed passage accuracy.
The adequacy of the test construction for the IAWL was important in establishing the validity of the measure.
When analyzing the overall measure, the IAWL assessment behaved as developed.
Despite the results demonstrating the overall effectiveness of IAWL's test construction, IRT analysis showed that the coding system of the words was not as accurate.
One limitation of the IAWL in this study is that it is specific to the PHAST intervention.
Finally, the use of the theoretically based IAWL is in the initial stages of development.