What is the difference between interval and ratio data?

Ratio data has all the properties of interval data, but it also has a true zero. Zero means that the measured quantity is completely absent.

For example:

Temperature (interval)

  • The difference between 10°C and 20°C is the same as the difference between 20°C and 30°C.
  • A temperature of 0°C doesn’t mean there is no temperature (no true zero).
  • You can’t say that 20°C is twice as hot as 10°C because there is no true zero.

Weight (ratio)

  • The difference between 10 kg and 20 kg is the same as the difference between 20 kg and 30 kg.
  • A weight of 0 kg means there is no weight.
  • Therefore, 20 kg is twice as heavy as 10 kg.