A Robot Installation Index Is Not a Unit Count: Reading a Base of 100

An indexed chart can make a robot installation series easier to compare across periods or markets, but an index of 120 is not a count of 120 robots. It is a value relative to a chosen reference period. Before using such a chart in a market report, record the base period, the underlying measure and whether different series use comparable definitions.

Understand what the base represents

Statistics Canada's archived guide to basic statistical techniques explains index numbers using a base assigned the value 100. For a simple quantity index, a value of 120 means the quantity is 20% above its base-period value. The index itself does not tell a reader the absolute number of units unless the original series or base quantity is also available.

For a hypothetical robotics series, a market with 1,000 installations in a base year and 1,200 in a later year has a simple index of 120 when the base is 100. These are invented numbers for illustration, not reported market data. The same index could describe a different market with 10,000 and 12,000 units, so equal index values do not imply equal market size.

Index points and percentage change are different

If an index rises from 120 to 132, it increases by 12 index points. The percentage increase over that interval is 10%, calculated relative to the earlier value of 120. Calling the change “12%” would confuse points with a percentage. Statistics Canada's consumer price index FAQ illustrates percentage changes between index values in its own price-statistics context.

The arithmetic can help interpret a simple robot installation index, but a price index and a count index measure different things. Do not apply the construction, weights or economic interpretation of a price index to robot installations without evidence that the series was built that way.

Keep the period and measure visible

Label the base year or period, each observation year and the underlying quantity. An index of annual installations is different from an index of operating stock or revenue. If a report uses a multi-year average as the base, say so rather than naming a single year. Preserve the source edition and any revision notes, because a revised base value can change an index series.

When comparing countries, check whether the same robot definition, sector coverage and time period apply. A market can grow faster from a small base while still installing fewer robots in absolute terms. An indexed chart is useful for comparing relative movement; a companion chart or table of absolute counts may be needed to answer a scale question.

Consider two hypothetical markets indexed to 100 in the same year. One rises to 150 and the other to 110. The first has grown more relative to its own starting value, but the chart does not reveal which market installed more robots. If the first began with 100 units and the second with 10,000, their later counts would be 150 and 11,000. These numbers are only an example of the arithmetic, not a claim about any actual country.

Rebasing changes the display, not the source history

A simple series can be expressed relative to a different nonzero reference value. If the original observations are unchanged and the calculation is consistent, the change between two observations remains the same even though the displayed index levels differ. Document the new reference and the formula, and retain the original data so another analyst can reproduce the result.

Do not silently rebase a chart and then compare its index levels with a series that uses another base. If the base is zero or missing, a simple ratio-based index cannot be calculated in the usual way. Mark the limitation and choose a different presentation instead of inventing a value.

A comparison should also indicate whether a series includes all industries or only a selected sector. Changes in classification, coverage or data collection can create apparent movement that is not directly comparable across a break. If the source documents such a break, mark it on the chart and explain its effect where known. Do not smooth over an unexplained change merely to create a continuous-looking trend.

Publish a checkable chart

Keep the original units, data source, extraction date, calculations and rounding rules with the chart. State whether each series is measured, estimated or forecast and whether the figures were revised. A reader should be able to distinguish a historical observation from a projection and a hypothetical demonstration from an actual dataset.