Function memtrend is used to calculate the two parameters for defining the
current influenza trend.
This method is based on the Moving Epidemics Method (MEM) used to monitor influenza
activity in a weekly surveillance system.
Usage
memtrend(i.data, i.seasons = 10)
Arguments
i.data
Data frame of input data.
i.seasons
Maximum number of seasons to use.
Details
Input data is a data frame containing rates that represent historical influenza surveillance
data. It can start and end at any given week (tipically at week 40th), and rates can be
expressed as per 100,000 inhabitants (or per consultations, if population is not
available) or any other scale.
The i.seasons parameter indicates how many seasons are used for calculating
thresholds. A value of -1 indicates the program to use as many as possible. If there
are less than this parameter, the program used all seasons avalaible.
There are three different states for trend, to determine the state, the current rate
and the difference of the current and last weekly rate are needed:
2
Ascending - When the weekly rate is above the epidemic threshold and
the difference of the current and last weekly rate is higher than Delta OR this is the
first time the rate is above the epidemic threshold.
3
Descending - When the weekly rate is above the epidemic threshold
and the difference of the current and last weekly rate is lower than Eta OR this is the
first time the rate is below the epidemic threshold after having been above it.
1
Stable - Otherwise.
Value
memtrend returns a list with two objects, the first one is the parameter used in
the calculations (param.seasons) and the second one (trend.thresholds) is
a matrix 1x2 with the Ascending (Delta) and Descending parameters (Eta).
1
Delta - Ascending parameter.
2
Eta - Descending parameter.
Author(s)
Jose E. Lozano Alonso <lozalojo@jcyl.es>.
References
Vega T., Lozano J.E. (2004) Modelling influenza epidemic - can we detect the beginning
and predict the intensity and duration? International Congress Series 1263 (2004)
281-283.
Vega T., Lozano J.E. (2012) Influenza surveillance in Europe: establishing epidemic
thresholds by the Moving Epidemic Method. Influenza and Other Respiratory Viruses,
DOI:10.1111/j.1750-2659.2012.00422.x.
Examples
## Castilla y Leon Influenza Rates data
data(flucyl)
## Finds the timing of the first season: 2001/2002
trend<-memtrend(flucyl)
trend
providing 
.