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Glossary page A

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Additive model (for time-series data)

A common approach to modelling time-series data *(Y)* in which it is assumed that the four components of a time series, *trend component (T)*, *seasonal component (S), cyclical component (C), and irregular component (I)*, are added to form the values of the time series at each time period.

In an additive model, the time series is expressed as: *Y = T + S + C + I.*

**Curriculum achievement objectives reference**

Statistical investigation: Level 8

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Association

A connection between two *variables*. Such a connection may not be evident until the *data* are displayed. An association between two variables is said to exist if the connection evident in a *data display* is so strong that it could not be explained as only due to *chance*.

In particular, two *numerical variables* are said to have **positive association** if the values of one variable tend to increase as the values of the other variable increase. Also two numerical variables are said to have **negative association** if the values of one variable tend to decrease as the values of the other variable increase.

See: *relationship*

**Curriculum achievement objectives references**

Statistical investigation: Levels 4, 5, 6, (7), (8)

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Average

A term used in two different ways.

When used generally, an average is a number that is representative or typical of the centre of a set of numerical values. In this sense, the number used could be the *mean* or the *median*. Sometimes the mode is used. This use of average has the same meaning as *measure of centre*.

When used precisely, the average is the number obtained by adding all values in a set of numerical values and then dividing this total by the number of values. This use of average has the same meaning as *mean*.

See:* measure of centre, mean, median, mode*

**Curriculum achievement objectives references**

Statistical investigation: Levels (5), (6), (7), (8)

Last updated October 9, 2013

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