Ratio scales, such as distance in inches, feet, yards, millimeters, meters, and so forth, start at zero and continue indefinitely. Zero means there is none of it and 20 means there is twice as much as 10. For example, the homicide rate is 3 per 10,000 persons in the city. Crime analysts will use nominal, ordinal, and ratio scales for these data but are very unlikely to use the fourth kind of measurement, interval. Interval scales show values but cannot show ratios between values.
Temperature is a good example.
A possible exception to the assertion that crime analysts will not use interval scales could be seriousness weighting . Thematic maps come in considerable variety and will be examined in more detail in chapter 2. Each type represents some kind of data best. Information with address-level detail calls for one kind of thematic map, whereas data measured only at the neighborhood, precinct, or census tract level require a different approach. Typically, the statistical symbols are placed in each subdivision on the map, such as patrol areas, census tracts, neighborhoods, or wards.