period of oscillations?

Question

Given the following graph, what is the period of oscillations?

http://i17.photobucket.com/albums/b58/asklepiades02/Image12.gif

Answer 1

12.25cycles/20sec = 0.6125 cycles/sec, = 0.6125Hz, which is the frequency. The period is the reciprocal of the frequency, which equals approx. 1.6327 seconds.

I didn't count that first half cycle because it looks longer than the rest. I started counting at the first positive zero-crossing at time 2.6, and counted 12.25 cycles from there to time 22.6

This is coming from someone who has made his living for the last few decades looking at and interpreting thousands of such displays.