It is generally assumed that self-report scales represent an interval scale of measurement. One could make reasonable arguments for anything from ordinal to ratio scales of measurement. For example, the argument for a ratio scale is the the self-report scale represents the number of items endorsed that represent whatever is being measured. So endorsing 10 items is endorsing twice as many items as endorsing five items. The argument for ordinal scales is that the underlying dimension (in this example, it is depression) has equal intervals, but there is no guarantee that the items of the scale tap depression precisely enough that the intervals between scores on the scale are equal. Faced with such ambiguity, the norm is to ASSUME that self-report measures are interval scales unless there is clear evidence that the scale is less than ordinal (i.e., clear evidence that the difference between scores on different points of the scale are not equal).