1. Ahn, L., Blum, M., Langford, J.: Telling Humans and Computers Apart Automatically. Communications of the ACM 47(2), 57–60 (2004)
2. Bursztein, E., Bethard, S., Fabry, C., Jurafsky, D., Mitchell, J.C.: How Good are Humans at Solving CAPTCHAs? A Large Scale Evaluation. Security and Privacy, 399–413 (2010)
3. Khalil, A., Abdallah, S., Soha, A., Hassan, H.: Script Familiarity and Its Effect on CAPTCHA Usability: An Experiment with Arab Participants. International Journal of Web Portals (IJWP) 4(2), 74–87 (2012)
4. Banday, M.T., Shah, N.A.: Image Flip CAPTCHA. ISeCure 1(2), 103–121 (2009)
5. Banday, M.T., Shah, N.A.: A Study of CAPTCHAs for Securing Web Services. IJSDIA 1(2), 66–74 (2009)