1. A computer-implemented method for identifying available combinations of attribute levels for a type of product, the method comprising: maintaining, in a computer memory storage system, data that specifies: multiple different attributes that are characteristic of a type of product and, for at least some of the attributes, corresponding attribute levels that represent different possible values of the attribute to which they correspond, including a first attribute that is characteristic of the type of product and multiple attribute levels for the first attribute that represent different possible values of the first attribute, and a second attribute that is characteristic of the type of product and that is different from the first attribute and multiple attribute levels for the second attribute that represent different possible values of the second attribute, and combinations of attribute levels for different attributes that are available in a set of actual product offerings of the product type; causing a display of the first attribute and attribute levels for the first attribute; receiving an indication of one or more attribute levels for the first attribute that a respondent identified as being acceptable attribute levels for the first attribute from among the displayed attribute levels for the first attribute; in response to receiving the indication of the one or more acceptable attribute levels for the first attribute, comparing the one or more acceptable attribute levels for the first attribute to the data that specifies the combinations of attribute levels for different attributes that are available in the set of actual product offerings of the product type; based on results of comparing the one or more acceptable attribute levels for the first attribute to the data that specifies the combinations of attribute levels for different attributes that are available in a set of actual product offerings of the product type, determining that, within the set of actual product offerings of the product type, at least one of the attribute levels for the second attribute is not available in combination with the one or more acceptable attribute levels for the first attribute; and as a consequence of having determined that the at least one attribute level for the second attribute is not available in combination with the one or more acceptable attribute levels for the first attribute, causing a display of an indication that the at least one attribute level for the second attribute is not available in combination with the one or more acceptable attribute levels for the first attribute.