USCALE= the user-scaled value of 1 logit = 1

Specifies the number of reported user-scaled units per logit. When USCALE=1 (or USCALE= is omitted) then all measures are in logits.

 

Negative USCALE= values, such as USCALE= -10

Higher row score -> lower row measure

Higher column score -> higher column measure

USCALE= does not change the direction of the Andrich thresholds, so

with positive USCALE=, Andrich thresholds are in the direction of items (columns)

with negative USCALE=, Andrich thresholds are in the direction of persons (rows)

 

Table 20 gives the UMEAN= and USCALE= values for a conversion that gives the measures a range of 0-100.

 

Example 1: You want to user-rescale 1 logit into 45.5 units, so that differences of -100, -50, 0, +50, +100 correspond to success rates of 10%, 25%, 50%, 75%, 90%:

USCALE = 45.5

 

Example 2: You want to reverse the measurement directions, because

a) the data matrix is transposed so that the 'items' are examinees and the 'persons' are test questions:

b) the data are scored "backwards", such as rank orders, where lower score value -> higher ability

USCALE = -1

KEYn=, RESCORE=, ISGROUPS= will still apply to the columns, not the rows, of the data matrix. Centering will still be on the column measures.

 

Example 3: You want to approximate the "probit" measures used in many statistical procedures.

 UPMEAN = 0  ; set the person sample mean to zero

 USCALE = 0.59  ; probits * 1.7 = logits

 

but if you want your person sample to approximate N(0,1):

 

1. first analysis:

UPMEAN=0

USCALE=1

look at the person S.D. in Table 3.1

 

2. second analysis:

UPMEAN=0

USCALE=1/person S.D. from 1.

 

Example 4: You want to report measures in "log-odds units to the base 10" (lods) instead of the standard "logits to the base e".

 USCALE=0.434294482 ; the conversion between "natural" and base-10 logarithms.

 

For more examples, and how to compute this by hand, see User-friendly rescaling