Table 3.
A summary of terminology for describing time series analyses conducted in infectious disease epidemiology research
| Term | Definition |
|---|---|
| Time series data | A set or a sample of time-referenced observations or records with an identified time period, cycle and unit (e.g. day, week, month extracted from a timestamp as YYYY:MM:DD:HH:mm) often illustrated by dot, line or needle plots with axes reflecting time and an outcome of interest. |
| Distribution of time series data | A general summary of frequencies in time-referenced data – i.e. how often an outcome of interest reaches a certain level with respect to time units (often illustrated with histograms and density plots). |
| Time series analyses | A collection of methods to describe, explain and predict temporal processes with time-referenced data for an outcome of interest. |
| Trend | General temporal behaviour in an outcome of interest that can exhibit steady incremental changes (linear) or varying incremental changes (non-linear) over time. |
| Season | An interval of time within one time cycle (typically 1 calendar year) defined by a specific biological, environmental, physical, physiological, or other property or feature in a biological or non-biological system [24]. |
| Seasonal pattern | A recurrence of periods in an outcome of interest with alternating values (e.g. high and low) over the course of a time cycle. |
| Seasonality | A systematic periodic fluctuation in an outcome of interest over the course of one cycle (typically 1 calendar year) as an observable property of a biological or non-biological system. |
| Seasonal curve | An analytical representation of seasonal periodic fluctuations in an outcome of interest within one time cycle (typically 1 calendar year). |
| Seasonality features | A set of measurable characteristics to describe seasonality and a seasonal curve within 1 year, including seasonal peak, nadir, intensity, duration, speed at which a seasonal curve reaches its peak and speed at which a seasonal curve declines to its nadir [17]. |
| Peak or nadir timing | A seasonality feature that represents times when a seasonal curve of an outcome reaches its maximum or minimum [17]. |
| Amplitude or intensity | A seasonality feature that represents the difference between seasonal peaks and nadirs [17]. |
| Duration | A seasonality feature that represents the time interval when incidence rises above a specified threshold [17]. |
Terms specify differences between time series data, distributions and analyses, as well as trends, season, seasonal patterns, seasonality and seasonality features.