I watched a video about levels of measurement. I really don't get the idea or concept of "true zero" in that context.
Shoe length in inches. The answer is ratio but I don't get it how a shoe length can be zero.
True, a shoe can't have length 0; but that's true of
any measurement on a ratio scale! (If a person's height or weight is zero, there is no person there.) The point he's making is that on a ratio scale, zero means what we expect it to mean:
nothing.
The measurement, therefore, is
proportional to the actual size, which is the reason for the name "ratio". For instance, when the number is doubled, it means the thing being measured has doubled. That is true for lengths: a shoe 40 cm long is twice as long as a shoe 20 cm long.
In contrast, on an interval scale, such as Celsius temperature, 0 doesn't mean nothing (no heat); it's just an arbitrary temperature to which we assign the number 0. The same is true of shoe sizes as labeled on the shoe box, where a size 0 might just be the smallest size they make. A size 10 shoe is not twice as long as a size 5 shoe.
Here is another explanation that may be helpful:
