Requirements:
- Merges the training and the test sets to create one data set.
- Extracts only the measurements on the mean and standard deviation for each measurement.
- Uses descriptive activity names to name the activities in the data set
- Appropriately labels the data set with descriptive variable names.
- From the data set in step 4, creates a second, independent tidy data set with the average of each variable for each activity and each subject.
df : data frame that meets the requirements 1, 2, 3 and 4. df$Activity - Activity (factor) that was performed in that sample. df$Subject - Identifier of the subject that performed the activity. df[3:length(df)] - mean and std variables from the original data.
df_mean: data frame that meets requirement 5. df_mean$Activity - Activity (factor) that was performed in that sample. df_mean$Subject - Identifier of the subject that performed the activity. df_mean[3:length(df)] - mean of the variables in df, grouped by activity-subject.