Compute 24 Measures Describing the Features of the Trajectories

Description

Computes 24 measures for each of the trajectories. See details for the list of measures.

Usage

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  step1measures(Data, Time, ID = FALSE, verbose = TRUE)

Arguments

Data

A n by m matrix or data frame containing the values of each individual trajectory. Each row corresponds to one of the n trajectories, while the m columns correspond to the ordered values of a given trajectory. See details

Time

A n by m matrix or data frame containing the measurement times corresponding to the values of the Data data frame. See details.

ID

Logical. Set to TRUE if the first column of Data corresponds to an ID variable. Defaults to FALSE

verbose

Logical. Set to TRUE to print information on screen. Defaults to TRUE

Details

There must be a minimum of 4 observations for each trajectory or the trajectory will be omitted from the analysis. The trajectories do not need to have the same number of observations, nor the same values of Time.

The Time data.frame or matrix must have the same dimension as the Data data frame or matrix and must not contain missing values or an error will be returned. The data can have missing values, Time can not.

When ID is set to FALSE, a generic ID variable is created and appended as the first colunm of both the Data and Time data.frames.

The 24 measures are:

1. Range
2. Mean-over-time*
3. Standard deviation (SD)
4. Coefficient of variation (CV)
5. Change
6. Mean change per unit time
7. Change relative to the first score
8. Change relative to the mean over time
9. Slope of the linear model*
10. R^2: Proportion of variance explained by the linear model
11. Maximum of the first differences
12. SD of the first differences
13. SD of the first differences per time unit
14. Mean of the absolute first differences*
15. Maximum of the absolute first differences
16. Ratio of the maximum absolute difference to the mean-over-time
17. Ratio of the maximum absolute first difference to the slope
18. Ratio of the SD of the first differences to the slope
19. Mean of the second differences
20. Mean of the absolute second differences
21. Maximum of the absolute second differences
22. Ration of the maximum absolute second difference to the mean-over-time
23. Ratio of the maximum absolute second difference to mean absolute first difference
24. Ratio of the mean absolute second difference to the mean absolute first difference

* If a measure is equal to zero, it will be set to the smallest, non-zero value of the same measure across the sample during further calculations. If Y_1, the first observation of the trajectory of an individual, is equal to zero, it will aslo be replaced.

For the exact equations of the measures, please go to "User guides, package vignettes and other documentation" section of the "traj" package.

Value

trajMeasures

Object containing the data used for the calculations and the 24 measures.

Author(s)

Marie-Pierre Sylvestre, Dan Vatnik
dan.vatnik@gmail.com

Examples

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# Setup data and time
data = example.data$data
time = example.data$time

# Run step1measures
s1 = step1measures(data,time, ID=TRUE)

# Display measures
head(s1$measurments)

# Plot mean trajectory of all individuals
plot(s1$measurments$ID, s1$measurments$m5)

# The next step would be to run "step2factors"