You incorrectly presume:
* The 100% loss of Skype is 100% loss of productivity.
* the use of Skype vs. other phone is either/or.

My assumption wasn’t 100% loss of Skype itself, but 100% loss of the host running Skype.

The statement that 10% of the users use Skype 100% of the time is merely to estimate 10% Skype usage of all calls. This is for simulation purposes only and is how I derive my numbers.

Clarity or lack thereof, notwithstanding, my calculations were intended to simulate Skype usage at 10% of all long distance calls and the downtime caused by a Skype-bourne worm or equivalent was a 10% productivity loss.

Calculating risk analysis such as this isn’t an exact science by any stretch of the imagination!

Although I am not a proponent of Skype in business until they implement more robust and simpler administrative controls, your calculations, although mathematically correct, do not paint a true picture.

Unless the staff is all telephone work, such as call center or similar, the complete loss of Skype will not completely cripple productivity.

Additionally, the deployment of emergency or replacement telephones when only 10% uses Skype implies simplicity. If 90% “still” uses old style phones, how hard is to “add” or turn back on for the 10% which is out of commission?

I have seen Skype used as office-to-office more often then 100% use. Users would default back to old telephone when the QoS dropped, without loss of productivity.

I mislabeled part of the first example as months when it should have been days.

Sorry about that.

That makes the score;

Enterprise: 2, Skype: 0.

What are the chances of a network-impacting Skype attack? And then one that affects more than just that stupid user? What about bandwidth costs (unless you’re already using VoIP)? Will this impact firewall logging/monitoring with the crazy connections it likes to open and if so what is that amount? And then balance all of that against the estimated value of improve morale?

Just some thoughts…obviously risk and cost can be made as complicated as you want, but I like this little exercise you posted up above.

I wonder how this would work with VoIP as a replacement to the regular phone system.