The Survey of Adult Skills locates items and individuals on the three proficiency scales using a response probability (RP) value of 0.67. In other words, individuals are located on the scale at the point at which he or she has a 67% probability of successfully completing a random set of items representing the construct measured. Items are located on the scale at the point at which they have a 67% probability of being successfully completed by a random sample of the adult population. This differs from the approach used in IALS and ALL in which a response probability of 0.80 was used. This change was made so that the approach used to define what it means for a person to be at a certain proficiency level was similar to that used in PISA (see OECD, 2010, p. 48).
The change in response probability has no consequences for either the estimation of the proficiency or the precision of the scales. The estimation of proficiency is independent of the selection of an RP value, as it is a function of the level of correct response to the test items. The precision of the scale is a function of the number of items in the scale, which is again independent of the choice of RP value. What the change in RP value does affect is the way proficiency is defined and described. In effect, “proficiency” is defined in terms of a different probability of successfully completing tasks. In the case of the shift from an RP value of 0.80 to one of 0.67, the result is that proficiency is described in terms of more difficult items that are completed with a lower probability of success.
This can be seen in the Table A.1 below, which presents item maps for literacy and numeracy when response probabilities of 0.67 and 0.80 are used. For example, the literacy item “Summer Streets” is located at 350 on the scale when a response probability of 0.67 is used as opposed to 369 when 0.80 is used. Similarly, the numeracy item “TV” moves from 279 to 260 when the response probability changes from 0.67 to 0.80.