Alpha is calculated as the excess return of the stock over and above the corresponding benchmark returns. The data item is calculated using 104 weekly price close points.
Benchmark is the standard against which performance of a security is measured. For example if one were to analyse the stock price performance of HDFC Bank over the previous year, just saying the stock moved up 17.3% will not give us the correct picture. It is important to compare the returns with the either Nifty 50 returns or Nifty Bank returns. Suppose we choose Nifty Bank as the benchmark, which has moved up 12% over the previous year. We can then conclude that price performance of HDFC Bank over the previous year has been better than the corresponding benchmark index. Alpha in this case is (17.3 – 12.0) = 5.3% .