The words behind the charts, with a simple example.

The ending value minus the starting value.

A worked example: From £100 to £120 is an absolute change of £20.

A reference line that maps values or time to positions on a chart.

A worked example: Ticks of 100, 110 and 120 are equally spaced on a linear axis.

The reference observation against which later values are compared.

A worked example: With a base of 80, a value of 92 has an index of 115.

A measure of how quantities vary together; it does not establish a cause.

A worked example: Two series rising together does not show that one caused the other.

The origin of the observations and their definitions.

A worked example: Our Compare Lab uses fixed illustrative data, not market quotes.

The first or last observation in a selected window.

A worked example: 100 and 110 are the endpoints of [100, 120, 80, 110].

How often observations are recorded.

A worked example: Monthly data provide one observation per month.

Values expressed relative to a chosen base, often set to 100.

A worked example: 230 divided by 200, multiplied by 100, gives 115.

An estimate of a value between known observations.

A worked example: 115 between 110 and 120 can be an estimate, but is not a measurement.

A scale where equal distances represent equal numerical differences.

A worked example: 100 to 110 occupies the same distance as 200 to 210.

A scale where equal distances represent equal ratios.

A worked example: 10 to 100 and 100 to 1,000 represent the same tenfold ratio.

The sum of the values divided by their number.

A worked example: The mean of 10, 11 and 12 is 11.

The middle ordered value, or the mean of the two middle values.

A worked example: The median of 10, 11, 12 and 50 is 11.5.

An observation that is unavailable, not necessarily zero.

A worked example: No March value does not mean the March value was 0.

An average recalculated over a specified sequence of observations.

A worked example: The three-point average of 10, 14 and 12 is 12.

A recorded value associated with a defined point or period.

A worked example: £110 at observation 2 is one value in illustrative series A.

An observation that is unusually far from the others in a stated context.

A worked example: 50 is unusual in [10, 11, 50, 12], but may still be valid.

The change divided by the starting value, multiplied by 100.

A worked example: From 80 to 100 is a 25% increase.

The unit for the arithmetic difference between two percentages.

A worked example: 20% to 25% is a rise of 5 percentage points.

The maximum minus the minimum in a set of observations.

A worked example: Values from 80 to 120 have a range of 40.

Enough information for someone else to repeat the calculation.

A worked example: Record the source, period, unit, values and formula.

The start and end of the interval being examined.

A worked example: Observations 2–4 form a different window from 1–4.

A broad direction over a specified period, not a prediction.

A worked example: A recent rise can sit within a longer decline.

What each numerical value measures.

A worked example: GBP and an index with base 100 are different units.