ActivityIndex package in RThe ActivityIndex
package contains functions to 1) read raw accelerometry data and 2)
compute “Activity Index” (AI) using the raw data. This introduction
provides step-by-step instructions on how to read data from
.csv files and then compute AI.
The sample data were collected by accelerometer GT3X+ (ActiGraph, ), downloaded from . The data are available in the ActivityIndex package and their paths can be acquired using command:
system.file("extdata","sample_GT3X+.csv.gz",package="ActivityIndex")
system.file("extdata","sample_table.csv.gz",package="ActivityIndex")sample_GT3X+.csv.gz is the standard output of GT3X+
accelerometer, with a 10-line header
containing the basic information of the data collection, followed by
3-column raw acceleration data.
sample_table.csv.gz contains the same 3-column acceleration data without the 10-line header. The first 15 lines of sample_GT3X+.csv.gz
are shown below:
## ------------ Data File Created By ActiGraph GT3X+ ActiLife v6.7.1 Firmware v2.5.0 date format M/d/yyyy at 30 Hz Filter Normal -----------
## Serial Number: NEO1DXXXXXXXX
## Start Time 10:54:00
## Start Date 6/27/2012
## Epoch Period (hh:mm:ss) 00:00:00
## Download Time 16:25:52
## Download Date 6/28/2012
## Current Memory Address: 0
## Current Battery Voltage: 4.22 Mode = 12
## --------------------------------------------------
## 0,0,0
## 0,0,0
## 0,0,0
## 0,0,0
## 0,0,0while the first 5 lines of
sample_table.csv.gz are
## 0,0,0
## 0,0,0
## 0,0,0
## 0,0,0
## 0,0,0Users should follow the same format while preparing their own data.
ReadGT3XPlus and ReadTable functions read
the GT3X+ .csv.gz file and the 3-column acceleration table, respectively. To
read the data, use the following code
sampleGT3XPlus=ReadGT3XPlus(system.file("extdata","sample_GT3X+.csv.gz",package="ActivityIndex"))
sampleTable=ReadTable(system.file("extdata", "sample_table.csv.gz",package="ActivityIndex"))Now that object sampleGT3XPlus has class
GT3XPlus, which contains the raw data and header
information. Function ReadGT3XPlus automatically applies
time stamps to the acceleration time series using the information from
the header. For example, our sample data look like this
## List of 8
## $ SN : chr "NEO1DXXXXXXXX"
## $ StartTime : chr "10:54:00"
## $ StartDate : chr "2012-06-27"
## $ Epoch : chr "00:00:00"
## $ DownloadTime: chr "16:25:52"
## $ DownloadDate: chr "2012-06-28"
## $ Hertz : num 30
## $ Raw :Classes 'data.table' and 'data.frame': 1006080 obs. of 5 variables:
## ..$ Date: chr [1:1006080] "2012-06-27" "2012-06-27" "2012-06-27" "2012-06-27" ...
## ..$ Time: chr [1:1006080] "10:54:00" "10:54:00" "10:54:00" "10:54:00" ...
## ..$ X : num [1:1006080] 0 0 0 0 0 0 0 0 0 0 ...
## ..$ Y : num [1:1006080] 0 0 0 0 0 0 0 0 0 0 ...
## ..$ Z : num [1:1006080] 0 0 0 0 0 0 0 0 0 0 ...
## ..- attr(*, ".internal.selfref")=<externalptr>
## - attr(*, "class")= chr "GT3XPlus"However, sampleTable is much simpler, since limited
information was given. The first 6
lines of it look like this
## Index X Y Z
## <int> <num> <num> <num>
## 1: 1 0 0 0
## 2: 2 0 0 0
## 3: 3 0 0 0
## 4: 4 0 0 0
## 5: 5 0 0 0
## 6: 6 0 0 0AI is a metric to reflect the variability of the raw acceleration signals after removing systematic noise of the signals. Formally, its definition (a one-second AI) is
$$ \text{AI}^{\text{new}}_i(t;H)=\sqrt{\max\left(\frac{1}{3}\left\{\sum_{m=1}^{3}{\frac{\sigma^2_{im}(t;H)-\bar{\sigma}^2_{i}}{\bar{\sigma}^2_{i}}}\right\},0\right)},\label{EQ: AI} $$ where σim2(t; H) (m = 1, 2, 3) is axis-m’s moving variance during the window starting from time t (of size H), and σ̄i is the systematic noise of the signal when the device is placed steady.
Function computeActivityIndex is used to compute AI. The
syntax of the function is
x is the data used to compute AI. It can either be a
GT3XPlus object, or a 4-column data frame (tri-axial acceleration
time series with an index column). Either x_sigma0 or
sigma0 are used to determine the systematic noise σ̄i. More
detailed example will follow to illustrate how to use them.
epoch is the epoch length (in second) of the AI. For
example, the default epoch=1 yields to 1-second AI, while minute-by-minute AI is
given by epoch=60. hertz specifies the sample
rate (in Hertz), which is usually 10,
30 or 80, etc.
We will continue our example of computing AI using our data
sampleGT3XPlus and sampleTable.
According to the definition of the systematic noise σ̄i, it changes
with subject i. Therefore,
strictly speaking, we are to compute σ̄i every time we
compute AI for a new subject i. Argument x_sigma0
can be used to specify a 4-column data
frame (one column for indices and three columns for acceleration) which
is used to calculate σ̄i. The 4-column data frame should contain the raw
accelerometry data collected while the accelerometer is not worn or kept
steady. For example, if we say a segment of our sample data
(sampleTable[1004700:1005600,]) meets such requirement, we
could compute AI using the following code
Sometimes we do not want to calculate σ̄i whenever computing AI. For example, if 10 accelerometers were used to collect data over 100 subjects, there is no reason to calculate σ̄i for 100 times. One σ̄i is only needed for one accelerometer. Furthermore, if we could verify the σ̄i’s of the 10 accelerometers are close to each others, we could combine them into a single $\bar{\sigma}=\sum_{i=1}^{10}{\bar{\sigma}_i}/10$. In this case, σ̄ will be used for all subjects in that study, which is crucial for fast processing of data collected by large studies.
This can be achieved by using the argument x_sigma0 to
specify a pre-determined σ̄i. Still using
the same segment of data (sampleTable[1004700:1005600,]) as
an example, we calculate a sample_sigma0 beforehand with
code
Then we could use this sample_sigma0=0.00218 to compute AI with code
Using either method to compute AI yield to the same result. The
output of function computeActivityIndex has two columns:
RecordNo saves the indices and AI stores AI.
The first 10 lines of
AI_sampleGT3XPlus is as follow
## Showing head and tail rows
## RecordNo AI
## 1 10:54:00 0.000
## 2 10:54:01 0.000
## 3 10:54:02 0.000
## 4 10:54:03 0.000
## 5 10:54:04 133.708
## 6 10:54:05 34.837
## RecordNo AI
## 5 10:54:04 133.708
## 6 10:54:05 34.837
## 7 10:54:06 62.947
## 8 10:54:07 54.207
## 9 10:54:08 124.708
## 10 10:54:09 147.842We could also compute AI in different epoch. Say we want minute-by-minute AI, then we could use the following code
AI_sampleTable_min=computeActivityIndex(sampleTable, sigma0=sample_sigma0, epoch=60, hertz=30)
AI_sampleGT3XPlus_min=computeActivityIndex(sampleGT3XPlus, sigma0=sample_sigma0, epoch=60, hertz=30)And according to the definition of AI, the minute-by-minute AI’s are simply the sum of all 1-second AI within each minute. The AI during the first 6 minutes are
## Showing head and tail rows
## RecordNo AI
## 1 10:54:00 3002.460
## 2 10:55:00 392.185
## 3 10:56:00 655.593
## 4 10:57:00 89.337
## 5 10:58:00 499.150
## 6 10:59:00 238.130
## RecordNo AI
## 1 10:54:00 3002.460
## 2 10:55:00 392.185
## 3 10:56:00 655.593
## 4 10:57:00 89.337
## 5 10:58:00 499.150
## 6 10:59:00 238.130