##Study design
The experiments have been carried out with a group of 30 volunteers within an age bracket of 19-48 years. Each person performed six activities (WALKING, WALKING_UPSTAIRS, WALKING_DOWNSTAIRS, SITTING, STANDING, LAYING) wearing a smartphone (Samsung Galaxy S II) on the waist. Using its embedded accelerometer and gyroscope, we captured 3-axial linear acceleration and 3-axial angular velocity at a constant rate of 50Hz. The experiments have been video-recorded to label the data manually. The obtained dataset has been randomly partitioned into two sets, where 70% of the volunteers was selected for generating the training data and 30% the test data.
The sensor signals (accelerometer and gyroscope) were pre-processed by applying noise filters and then sampled in fixed-width sliding windows of 2.56 sec and 50% overlap (128 readings/window). The sensor acceleration signal, which has gravitational and body motion components, was separated using a Butterworth low-pass filter into body acceleration and gravity. The gravitational force is assumed to have only low frequency components, therefore a filter with 0.3 Hz cutoff frequency was used. From each window, a vector of features was obtained by calculating variables from the time and frequency domain. See 'features_info.txt' for more details.
##Feature Selection
The features selected for this database come from the accelerometer and gyroscope 3-axial raw signals tAcc-XYZ and tGyro-XYZ. These time domain signals (prefix 't' to denote time) were captured at a constant rate of 50 Hz. Then they were filtered using a median filter and a 3rd order low pass Butterworth filter with a corner frequency of 20 Hz to remove noise. Similarly, the acceleration signal was then separated into body and gravity acceleration signals (tBodyAcc-XYZ and tGravityAcc-XYZ) using another low pass Butterworth filter with a corner frequency of 0.3 Hz.
Subsequently, the body linear acceleration and angular velocity were derived in time to obtain Jerk signals (tBodyAccJerk-XYZ and tBodyGyroJerk-XYZ). Also the magnitude of these three-dimensional signals were calculated using the Euclidean norm (tBodyAccMag, tGravityAccMag, tBodyAccJerkMag, tBodyGyroMag, tBodyGyroJerkMag).
Finally a Fast Fourier Transform (FFT) was applied to some of these signals producing fBodyAcc-XYZ, fBodyAccJerk-XYZ, fBodyGyro-XYZ, fBodyAccJerkMag, fBodyGyroMag, fBodyGyroJerkMag. (Note the 'f' to indicate frequency domain signals).
##Description variables
All measures values are the means of each activity and subject, this measures were before Normalized and bounded within [-1,1]
| variable | description | values |
|---|---|---|
| activity | Activity performed wearing a smartphone (Samsung Galaxy S II) on the waist | LAYING, SITTING, STANDING, WALKING WALKING_DOWNSTAIRS |
| subject | The subject ID who performed the experiment. | [1,30] |
| tBodyAcc-mean-X | Mean time for acceleration of body for X direction | [-1,1] |
| tBodyAcc-mean-Y | Mean time for acceleration of body for Y direction | [-1,1] |
| tBodyAcc-mean-Z | Mean time for acceleration of body for Z direction | [-1,1] |
| tBodyAcc-std-X | Standard deviation of time for acceleration of body for X direction | [-1,1] |
| tBodyAcc-std-Y | Standard deviation of time for acceleration of body for Y direction | [-1,1] |
| tBodyAcc-std-Z | Standard deviation of time for acceleration of body for Z direction | [-1,1] |
| tGravityAcc-mean-X | Mean time of acceleration of gravity for X direction | [-1,1] |
| tGravityAcc-mean-Y | Mean time of acceleration of gravity for Y direction | [-1,1] |
| tGravityAcc-mean-Z | Mean time of acceleration of gravity for Z direction | [-1,1] |
| tGravityAcc-std-X | Standard deviation of time of acceleration of gravity for X direction | [-1,1] |
| tGravityAcc-std-Y | Standard deviation of time of acceleration of gravity for Y direction | [-1,1] |
| tGravityAcc-std-Z | Standard deviation of time of acceleration of gravity for Z direction | [-1,1] |
| tBodyAccJerk-mean-X | Mean time of body acceleration jerk for X direction | [-1,1] |
| tBodyAccJerk-mean-Y | Mean time of body acceleration jerk for Y direction | [-1,1] |
| tBodyAccJerk-mean-Z | Mean time of body acceleration jerk for Z direction | [-1,1] |
| tBodyAccJerk-std-X | Standard deviation of time of body acceleration jerk for X direction | [-1,1] |
| tBodyAccJerk-std-Y | Standard deviation of time of body acceleration jerk for Y direction | [-1,1] |
| tBodyAccJerk-std-Z | Standard deviation of time of body acceleration jerk for Z direction | [-1,1] |
| tBodyGyro-mean-X | Mean body gyroscope measurement for X direction | [-1,1] |
| tBodyGyro-mean-Y | Mean body gyroscope measurement for Y direction | [-1,1] |
| tBodyGyro-mean-Z | Mean body gyroscope measurement for Z direction | [-1,1] |
| tBodyGyro-std-X | Standard deviation of body gyroscope measurement for X direction | [-1,1] |
| tBodyGyro-std-Y | Standard deviation of body gyroscope measurement for Y direction | [-1,1] |
| tBodyGyro-std-Z | Standard deviation of body gyroscope measurement for Z direction | [-1,1] |
| tBodyGyroJerk-mean-X | Mean jerk signal of body for X direction | [-1,1] |
| tBodyGyroJerk-mean-Y | Mean jerk signal of body for Y direction | [-1,1] |
| tBodyGyroJerk-mean-Z | Mean jerk signal of body for Z direction | [-1,1] |
| tBodyGyroJerk-std-X | Standard deviation of jerk signal of body for X direction | [-1,1] |
| tBodyGyroJerk-std-Y | Standard deviation of jerk signal of body for Y direction | [-1,1] |
| tBodyGyroJerk-std-Z | Standard deviation of jerk signal of body for Z direction | [-1,1] |
| tBodyAccMag-mean | Mean magnitude of body Acc | [-1,1] |
| tBodyAccMag-std | Standard deviation of magnitude of body Acc | [-1,1] |
| tGravityAccMag-mean | Mean gravity acceleration magnitude | [-1,1] |
| tGravityAccMag-std | Standard deviation of gravity acceleration magnitude | [-1,1] |
| tBodyAccJerkMag-mean | Mean magnitude of body acceleration jerk | [-1,1] |
| tBodyAccJerkMag-std | Standard deviation of magnitude of body acceleration jerk | [-1,1] |
| tBodyGyroMag-mean | Mean magnitude of body gyroscope measurement | [-1,1] |
| tBodyGyroMag-std | Standard deviation of magnitude of body gyroscope measurement | [-1,1] |
| tBodyGyroJerkMag-mean | Mean magnitude of body body gyroscope jerk measurement | [-1,1] |
| tBodyGyroJerkMag-std | Standard deviation of magnitude of body body gyroscope jerk measurement | [-1,1] |
| fBodyAcc-mean-X | Mean frequency of body acceleration for X direction | [-1,1] |
| fBodyAcc-mean-Y | Mean frequency of body acceleration for Y direction | [-1,1] |
| fBodyAcc-mean-Z | Mean frequency of body acceleration for Z direction | [-1,1] |
| fBodyAcc-std-X | Standard deviation of frequency of body acceleration for X direction | [-1,1] |
| fBodyAcc-std-Y | Standard deviation of frequency of body acceleration for Y direction | [-1,1] |
| fBodyAcc-std-Z | Standard deviation of frequency of body acceleration for Z direction | [-1,1] |
| fBodyAccJerk-mean-X | Mean frequency of body accerlation jerk for X direction | [-1,1] |
| fBodyAccJerk-mean-Y | Mean frequency of body accerlation jerk for Y direction | [-1,1] |
| fBodyAccJerk-mean-Z | Mean frequency of body accerlation jerk for Z direction | [-1,1] |
| fBodyAccJerk-std-X | Standard deviation frequency of body accerlation jerk for X direction | [-1,1] |
| fBodyAccJerk-std-Y | Standard deviation frequency of body accerlation jerk for Y direction | [-1,1] |
| fBodyAccJerk-std-Z | Standard deviation frequency of body accerlation jerk for Z direction | [-1,1] |
| fBodyGyro-mean-X | Mean frequency of body gyroscope measurement for X direction | [-1,1] |
| fBodyGyro-mean-Y | Mean frequency of body gyroscope measurement for Y direction | [-1,1] |
| fBodyGyro-mean-Z | Mean frequency of body gyroscope measurement for Z direction | [-1,1] |
| fBodyGyro-std-X | Standard deviation frequency of body gyroscope measurement for X direction | [-1,1] |
| fBodyGyro-std-Y | Standard deviation frequency of body gyroscope measurement for Y direction | [-1,1] |
| fBodyGyro-std-Z | Standard deviation frequency of body gyroscope measurement for Z direction | [-1,1] |
| fBodyAccMag-mean | Mean frequency of body acceleration magnitude | [-1,1] |
| fBodyAccMag-std | Standard deviation of frequency of body acceleration magnitude | [-1,1] |
| fBodyBodyAccJerkMag-mean | Mean frequency of body acceleration jerk magnitude | [-1,1] |
| fBodyBodyAccJerkMag-std | Standard deviation of frequency of body acceleration jerk magnitude | [-1,1] |
| fBodyBodyGyroMag-mean | Mean frequency of magnitude of body gyroscope measurement | [-1,1] |
| fBodyBodyGyroMag-std | Standard deviation of frequency of magnitude of body gyroscope measurement | [-1,1] |
| fBodyBodyGyroJerkMag-mean | Mean frequency of magnitude of body gyroscope jerk measurement | [-1,1] |
| fBodyBodyGyroJerkMag-std | Standard deviation frequency of magnitude of body gyroscope jerk measurement | [-1,1] |