I'm a high school science teacher, my primary degree is in Physics, so I have a solid grasp of the background. I'm running into a strange issue with a pendulum lab I had my students complete.
We're using iOLab devices (http://www.iolab.science/) as our pendulum and I'm having the students measure the time by reading the $\Delta t$ from the graph generated by the software. We set the software to record the accelerometer data, and the y-axis data makes a nice sine wave that corresponds to the motion of the pendulum. I've tested it with the force sensor as well, and the same wave appears. To reduce errors, I instructed the students to find the $\Delta t$ for three complete waves, and then we divide by three, to average them.
I tested it myself trying to eliminate errors, and I've encountered the same following issue as the students. When we plug the length (in $m$) and the period (in $s$) into the equation for the period of a pendulum (solved algebraically for the acceleration due to gravity), we get a value approximately 4 times higher than the known value. I've checked and double-checked the algebra, and verified that I'm entering the numbers correctly into the calculation. When I plugged the length and the known value for the acceleration due to gravity into the equation to find the period, I got a calculated period that was about 3 times what we measured.
I cannot figure out anything that could be causing that much of a difference, unless the acceleration due to gravity at our school is different than the rest of the planet.