Data elements can always be reduced in their level of measurement but can never be increased. For example, if income
is measured at the ratio level (in exact dollar amounts), the data elements can be reduced to ordinal level data by the creation of categories (Table 1). However, data measured at the ordinal level of measurement cannot be increased to the ratio level of measurement. If we ask a subject what category her income fits into, we can never determine from the raw data her exact income level. Consequently, data should always be collected at the highest level of measurement possible and converted at the time of data analysis if a lower level of measurement is desired. This recommendation does not hold
when there is strong reason to believe that a subject will not be truthful if she is required to provide exact data or when
the data cannot be expected to be accurately measured at the
higher level of measurement.