Calculate power spectrum with the time series data (as shown in the link below)

http://www.mathworks.com/help/signal/ug/psd-estimate-using-fft.html

Then calculate the area under the curve from 8Hz - 15Hz.(alpha band)

Hi Udeshika,

here are two useful references that can also help you to rationalize on how to compute individual alpha frequency range using EEGLab toolbox on Matlab and the channel reactivity based method. This toolbox might help to overpass your "issue" regarding the average power spectrum computing, giving the fact that it provides a simple interface to compute individual alpha frequency ranges. 

Goljahani et al. 2012; Goljahani et al. 2014 (on the following links) :

http://www.ncbi.nlm.nih.gov/pubmed/24439522

http://www.ncbi.nlm.nih.gov/pubmed/22182767

However, be aware that this computing method and the related findings are largely dependent of some parameters that you need to carefully select based on your hypothesis and experimental set up : channels to applied the CRB method, baseline window and window dataset to analyze, and up to 5 parameters of regulation for the spectral analysis.

Hope this help you,

Best regards

https://www.researchgate.net/post/Does_anyone_knows_how_I_can_quantify_the_individual_alpha_frequency_IAF

1)devide the  long data set into some short segments with the same length.

2)FFT for each short segment

3)average across all the FFT

Thank you very much for your comments.