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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

## 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)

## 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