ABSTRACT
It w o u l d b e d i f f i cu l t t o d e n y the c l a i m that the m o s t c o n t e n t i o u s i ssue i n R a s c h m e a s u r e m e n t c i rc l es is that o f fit. O u t s i d e R a s c h m e a s u r e m e n t c i r c l es , w e c o n - t i n u e t o p e r p l e x the rest o f the p s y c h o m e t r i c w o r l d b y i n s i s t i n g that o u r task is t o p r o d u c e da ta that fit the R a s c h m o d e l ' s spec i f i c a t i on ra the r t h a n ta lk o f fit i n w h a t o the rs c o n s i d e r t o b e the c o n v e n t i o n a l w a y (i.e., h o w w e l l the m o d e l fits the data ) . O f c ou r s e , the c o n c e p t o f fit m u s t be c o n s i d e r e d h a n d - i n - h a n d w i t h that o f u n i d i - mens iona l i t y . T h e c o n c e p t o f u n i d i m e n s i o n a l i t y ref lects the R a s c h m o d e l ' s f o cus o n the p r o c e s s o f f u n d a m e n t a l m e a s u r e m e n t , a n d it is essent ia l that o u r da ta fit the m o d e l i n o r d e r t o achieve inva r i an t m e a s u r e m e n t w i t h i n the m o d e l ' s u n i d i - m e n s i o n a l f r amework . I n d e e d , the benef i ts a n d p r o p e r t i e s o f R a s c h m e a s u r e m e n t ex is t o n l y t o the ex ten t that the da ta fit the m o d e l ' s d e m a n d i n g r e q u i r e m e n t s . T h i s is c ons i s t en t w i t h the p o s i t i o n e m p h a s i z e d t h r o u g h o u t th is text: e v e n i n the m o s t c o m p l e x m e a s u r e m e n t s i tua t i ons , i n d i v i d u a l a t t r ibutes s h o u l d b e m e a s u r e d o n e at a t ime .
