AP Syllabus focus: ‘Income levels and poverty rates vary widely across groups and countries.’
Measuring inequality means describing how unevenly income and wealth are distributed and how many people fall below a poverty threshold. Clear measurement choices shape what comparisons across groups and countries actually mean.
Core concepts and what is being measured
Income vs. wealth
Income and wealth capture different aspects of economic well-being and can show different inequality patterns.
Income: A flow of earnings received over a period of time (such as wages, rent, interest, and transfers), typically measured per month or per year.
Wealth is typically more unequal than income because it accumulates over time and can generate additional income.
Wealth (net worth): A stock of assets owned at a point in time minus liabilities (debts).
Poverty as a related outcome
Poverty measures focus on the lower tail of the income distribution and help interpret statements like “income levels and poverty rates vary widely across groups and countries.”
Poverty rate (headcount ratio): The percentage of people (or households) with income below a defined poverty line.
How income inequality is commonly measured (without a single index)
AP Microeconomics commonly emphasises interpreting reported statistics and comparisons.
Describing the distribution
Mean vs. median income
Median income is the middle value and is less affected by extreme high incomes.
Mean income can rise even if typical households do not benefit, when top incomes increase.
Income shares by group
Shares received by the bottom 20% (quintile), top 10%, or top 1% show concentration.
Percentile or quintile ratios
A Lorenz curve plots the cumulative share of income earned against the cumulative share of the population (ranked from lowest to highest income). The 45-degree line represents perfect equality, and the distance between the equality line and the Lorenz curve illustrates how concentrated income is. The shaded areas labeled A and B show the area-ratio logic behind the Gini coefficient (greater area A implies greater inequality). Source
Comparisons like the income at the 90th percentile relative to the 10th percentile summarise dispersion.
Choosing the income concept
Different definitions change measured inequality:
Market (pre-tax, pre-transfer) income vs. disposable (after-tax, after-transfer) income
Individual income vs. household income (households share resources)
Nominal vs. real income (cost of living and inflation matter for comparisons over time)
This bar chart shows regional price parities by U.S. state (index with 100 as the national average), illustrating how the cost of living varies substantially by location. It provides an intuitive reason economists convert nominal income into real (price-adjusted) income before comparing living standards across places. In inequality and poverty analysis, these adjustments help avoid falsely concluding that two regions with the same dollar income offer the same purchasing power. Source
Adjusting for household size (equivalised income) to compare living standards more fairly
How wealth inequality is measured
Wealth data focus on ownership of assets and debts; interpretation requires care because asset values fluctuate.
Key components
Assets: housing, retirement accounts, businesses, stocks and bonds, bank deposits
Liabilities: mortgages, student loans, credit card debt
Net worth distribution: how many households have near-zero or negative net worth versus very high net worth
Common summaries
Median wealth versus mean wealth (wealth inequality makes the mean especially sensitive to the top)
Top wealth shares (e.g., share held by top 10% of households)
Asset composition differences across groups (for instance, whether wealth is concentrated in home equity versus financial assets)
Comparing inequality across groups and countries
Comparisons rely on consistent definitions and data.
Across groups within a country: compare income levels and poverty rates by region, age, education, occupation, race/ethnicity, or household type (using the same income definition).
Across countries: ensure similar poverty thresholds and price adjustments; differences in tax and transfer systems can strongly affect disposable-income inequality.
Over time: use real, inflation-adjusted series and consistent survey methods to avoid false trends.
Measurement challenges and limitations
Underreporting and missing data: high incomes/wealth can be undercounted in surveys; informal earnings may be missed.
One-time vs. long-run perspective: annual income can vary temporarily (schooling, unemployment), so a snapshot may not reflect long-run living standards.
Non-cash benefits: employer health insurance or in-kind government benefits may not be fully captured in income measures.
Wealth valuation: housing and business assets are hard to price; market swings can change measured wealth inequality even without new saving.
FAQ
A relative line is tied to a country’s typical income (e.g., a fraction of median income), while an absolute line is a fixed real standard.
Relative poverty can rise even if living standards improve.
Households pool resources and share fixed costs (housing, utilities).
Per-person measures can overstate inequality if larger households are concentrated at particular income levels.
It rescales household income to reflect household size and composition.
This improves comparability of living standards across singles, couples, and families with children.
Assets like privately held businesses are difficult to value.
High-wealth households are less likely to respond to surveys, and some wealth is held in hard-to-track forms.
A single national poverty line ignores regional price differences.
Real purchasing power for the same nominal income can vary substantially across cities and rural areas.
Practice Questions
Define “wealth” and state one reason wealth inequality can be greater than income inequality. (2 marks)
Correct definition of wealth as assets minus liabilities at a point in time. (1)
One valid reason (e.g., accumulation over time; compounding returns; inheritances) stated. (1)
A government reports that median income rose while the poverty rate also rose. Explain how both can occur and identify two measurement choices that could affect these statistics. (5 marks)
Explanation consistent with distributional change (e.g., gains around the middle while the bottom falls; poverty line changes; rising costs for low-income households). (2)
Identifies two measurement choices (any two): real vs nominal; household vs individual; pre-tax vs disposable; equivalising for household size; poverty line definition (absolute vs relative); inclusion of transfers/non-cash benefits. (2)
Clear linkage of at least one choice to the reported outcomes. (1)
