How to check the baseline behind a percentage claim

The short answer

check the baseline behind a percentage claim

A percentage without a clearly stated baseline is incomplete. To assess any percentage-based claim made in political debate, you must answer four questions: percentage of what, compared with which baseline date, measured over what time period, and applied to which population. A claim that crime has increased by 20 per cent might look alarming until you discover the baseline was a historically low year during pandemic lockdowns. A claim that government spending rose by 5 per cent may appear modest until you learn it was adjusted for inflation and population growth, and the raw increase was substantially higher. The Australian Bureau of Statistics publishes the underlying data for most official numbers used in political claims, and the federal budget papers contain the detailed estimates behind government expenditure figures. Checking the baseline is about whether the number fairly represents the underlying trend.

This is a method for assessing public claims, not a verdict on a party or a direction on how to vote. Conclusions should change when the underlying law, data or implementation evidence changes.

The useful question is not only “what is the rule?” but also “who administers it, which document controls it, and when might it change?” That distinction prevents an accurate general explanation from becoming wrong advice in a particular election, chamber or policy setting.

Evidence review

Percentage of what: identifying the numerator

Every percentage is a ratio of two numbers. The numerator is the quantity being measured, and the denominator provides the comparison. When you hear a claim like unemployment has fallen by 15 per cent, your first question should be: 15 per cent of what? The unemployment rate itself is already a percentage, and a change in the rate from 4 per cent to 3.4 per cent is a decline of 0.6 percentage points, not 15 per cent. The 15 per cent figure likely refers to the count of unemployed persons, not the rate. Similarly, a claim that health funding has grown by 50 per cent may select a narrow program increase while omitting that total health expenditure rose by far less. Ask whether the percentage is applied to a narrow or broad measure, and whether the numerator includes factors that should be stripped out to match the headline claim.

Evidence review

Compared with when: selecting the baseline

The choice of baseline date can dramatically alter a percentage claim. A 10-year comparison may start from a trough or a peak, deliberately or otherwise. The COVID-19 pandemic created extreme one-off statistical movements in many data series. A claim that wages grew by 20 per cent since 2020 is less impressive than it sounds because 2020 was a period of suppressed wage growth. Where possible, check the same percentage claim against multiple baseline periods: one year, five years, ten years, to see whether the direction and magnitude are consistent. Also check whether seasonal adjustment has been applied. Quarterly data may be compared year-on-year, quarter-on-quarter, or in trend terms, each yielding different percentage changes. Budget papers also distinguish between changes measured from the previous budget's estimates and changes measured from actual outcomes, which can be materially different.

Evidence review

What population and what adjustments

A percentage applied to the total Australian population means something different from the same percentage applied to working-age adults, households or people in a specific income bracket. A claim about tax cuts may apply the percentage change to the total tax payable by a hypothetical family on a median income, not to all taxpayers. Inflation adjustment matters: a 10 per cent nominal increase over a period of 5 per cent cumulative inflation is only about a 5 per cent real increase. The ABS produces the Consumer Price Index and the Wage Price Index, along with chain volume measures in the national accounts, specifically to facilitate real comparisons. Government budget figures often distinguish between nominal and real growth, and between total expenditure and expenditure as a proportion of Gross Domestic Product.

Evidence review

Direction and composition: what the percentage conceals

A percentage change tells you the net movement but not what happened underneath. A 2 per cent increase in employment might conceal 5 per cent growth in part-time work and a decline in full-time positions. The ABS Labour Force publication breaks employment changes into full-time and part-time components, by gender, and by state and industry. Similarly, a budget measure that reduces spending growth by 1 per cent per year is different from a cut of 1 per cent. The first means spending still increases but more slowly; the second means it falls. Percentages based on small numbers are volatile and may not be statistically significant. The ABS publishes the relative standard error for survey-based estimates so users can assess whether a change is likely to be real or within the expected sampling variability.

Evidence review

Official sources to verify baseline claims

The Australian Bureau of Statistics is the authoritative source for most economic and social statistics used in political debate. Its website at abs.gov.au provides free access to all statistical releases, time series spreadsheets and methodological notes. The budget.gov.au website publishes the complete budget papers, including Budget Strategy and Outlook, which contains the detailed economic and fiscal forecasts and historical tables used by Treasury. The Parliamentary Budget Office publishes independent costings of election commitments and can be consulted to verify the fiscal impact of specific policy proposals. The Australian Institute of Health and Welfare publishes data on health, welfare and housing. When a political claim relies on ABS data, the source catalogue number and table reference are usually cited, or can be identified by searching the ABS website for the relevant release.

Evidence review

A step-by-step checklist for any percentage claim

First, write down the exact claim as stated. Second, identify what is being measured, the numerator. Third, identify the comparison group or period, the denominator and baseline. Fourth, check whether the number is nominal or real, and whether it accounts for population growth. Fifth, find the original data source by searching ABS or budget.gov.au for the underlying series. Sixth, compare the claim against alternative baselines and measures to see whether the percentage is robust or cherry-picked. Seventh, note what is not disclosed: the composition, the volatility, the uncertainty. This process takes minutes and does not require statistical training. It simply asks the speaker to show their working. A percentage is a rhetorical device that collapses a complex reality into a single number, and the only way to assess whether it is meaningful is to look at what was collapsed.

Common questions

Before you rely on the answer