A zip code is written in digits, yet it measures nothing. Classify each variable one question at a time, and find out which numbers are really labels.
Classification map
Every variable is qualitative or quantitative. Qualitative variables come in two types: ordinal variables and nominal variables. Quantitative variables are discrete or continuous, and they also have a level of measurement: ratio or interval. A variable written with digits is not always quantitative; a zip code or a jersey number is a label.