IPA IP 2025 Question 63

Time of day — an interval scale has equal intervals between values but no true zero, so differences mean something and ratios do not.

The four scales, from weakest to strongest:

• •Nominal — labels that only tell categories apart (blood type)
• •Ordinal — categories with an order, but the gaps between them are not equal quantities (school year)
• •Interval — equal steps, so subtraction makes sense, but zero is a chosen point, not an absence (time of day, temperature in °C)
• •Ratio — equal steps and a true zero, so ratios make sense (sleep time, weight)

The difference between 9:00 and 11:00 is two hours wherever on the clock it is measured, but 0:00 does not mean "no time", and 10:00 is not twice 5:00. So time of day is an interval scale.

Answer (c)

Why not others:

• •(a) — school year has an order (first year, second year…) but is used as a rank, an ordinal scale
• •(b) — blood types A, B, O and AB are categories with no order, a nominal scale
• •(d) — a length of sleep has a true zero (no sleep), and 8 hours is twice 4 hours, a ratio scale

Key rule: Interval scale = differences meaningful, zero arbitrary (clock time, °C); once zero means none and ratios work, it is a ratio scale (durations, lengths, weights).

Basic Theory · AI-written, may contain errors