What type of measurement includes fixed units but lacks an absolute zero point?

The type of measurement that includes fixed units but lacks an absolute zero point is interval measurement. Interval scales provide a consistent scale that measures the difference between values, meaning the intervals are equal. However, unlike ratio scales, interval scales do not have a true zero point that represents the absence of the quantity measured. A classic example of an interval measurement is the Celsius temperature scale; while 0 degrees Celsius is a specific point, it does not indicate the absence of temperature.

This distinction is crucial because, with interval measurements, one can add and subtract values, allowing for the analysis of differences between measurements. However, multiplicative operations, like calculating ratios, are not meaningful because the zero point is arbitrary. In contrast, ratio measurements do include a true zero, which allows for all arithmetic operations, including multiplication and division. Nominal measurements categorize items without any inherent order, and ordinal measurements rank items but do not provide equal intervals between them, thus lacking the fixed units that characterize interval scales.