Levels of measurement (Stevens' typology):

• Nominal scale categorizes data qualitatively without numeric ranking (e.g. gender, blood group).
• Ordinal scale establishes rank order without equal distances between points (e.g. grading, customer satisfaction).
• Interval scale has ordered units with equal intervals but an arbitrary zero point (e.g. Celsius temperature).
• Ratio scale possesses all interval properties plus a true absolute zero point (a salary of zero represents total absence of income, and $\$100,000$ is twice $\$50,000$). Hence, salaries are measured on a ratio scale.

Graphical determination of median:

• Arithmetic mean cannot be directly determined by the intersection of cumulative frequency curves.
• Geometric mean is not located via ogives.
• Harmonic mean is not located via ogives.
• The 'less than' ogive rises from $0$ to $N$, while the 'more than' ogive falls from $N$ to $0$. They intersect precisely at cumulative frequency $N/2$. Projecting this intersection point onto the horizontal X-axis gives the median.