Discrete Wavelet Transform Features Extraction

Hi. I am currently doing my project regarding Brain Computer Interfaces. The obtain EEG signal will undergoes a series of signal processing (Pre-processing-Features Extraction-Classification). May i know how can I obtain the average and standard deviation of a sub-band after DWT? My code as below but couldn't work.

Your help will be highly appreciated.

%Load the EEG signal (Sampling Frequency = 128Hz, 14 channel)
E1=load ('E:\Backup_Harddisk\Documents\David FYP References\PSM 1\Experiment\EEG-SSVEP-Experiment3\U001bi.mat');
%Select channel O2 and 1920 samples (15s long data)
SSS1=E1.eeg(3,1281:3200); 
%Applying bandpass filter to filter out the unwanted signal <4 and >30Hz
yy1=bandpass(SSS1,[4 30],128);
%Apply DWT at 5 level of decomposition
waveletFunction = 'db2';
[C,L] = wavedec(yy1,5,waveletFunction);
cD11 = detcoef(C,L,1);                   
cD21 = detcoef(C,L,2);                  
cD31 = detcoef(C,L,3);                   
cD41 = detcoef(C,L,4);                  
cD51 = detcoef(C,L,5);                  
cA51 = appcoef(C,L,waveletFunction,5);   

D11 = wrcoef('d',C,L,waveletFunction,1); %Gamma
D21 = wrcoef('d',C,L,waveletFunction,2); %Beta
D31 = wrcoef('d',C,L,waveletFunction,3); %Alpha
D41 = wrcoef('d',C,L,waveletFunction,4); 
D51 = wrcoef('d',C,L,waveletFunction,5); 
A51 = wrcoef('a',C,L,waveletFunction,5); 

%Find the mean
av1 = mean(D31);
%Find the standard deviation
sdA1 = std(D31);