How to quickly estimate the person's level of stress with a mobile based relaxation app? I'm writing a simple iPhone relaxation app and am thinking of a very quick test I can do to learn something about the user's level of stress, anxiety, etc. The objective is to measure how effective the app is or to adjust the length of the session. 

I thought of using the following measurement process:


Ask the user to trace a line with their finger, prior to relaxation
Repeat the tracing process following relaxation
Analyze and compare results to the user's previous entry or historic entries for the same user


Does this approach have any merit or validity? What other simple and quick ways are there to gauge how relaxed or stressed the user is? 

A mobile device, like iPhone has 3 axis accelerometer, gyroscope, magnetometer, and a capacitive touchscreen that does not measure the pressure of a touch.
Anxiety and stress are very available states, and since you want to know how people currently feel (rather than some more implicit construct), you probably can't do much better than just asking them. A single-item anxiety measure as suggested by mlt should work.