Projectile Motion Experimental Error

[ELLE_AW]

Homework Statement

I got an experimental vertical acceleration of -12 m/s^2 of projectile motion for an experiment I did at home where I just had to throw a ball at around 45 degree to the horizontal up in the air and record it's motion, then analyze the motion via computer software. Obviously the acceleration I was supposed to get was -9.80m/s^2. I did everything right and all my numbers make sense otherwise, and all my graphs look the way they're supposed to (x vs t graph looks linear) and (y vs t graph looks parabolic). What were some potential sources of error that I may have over-looked?

Homework Equations

acceleration due to gravity = -9.80 m/s^2
projectile motion (neglecting air resistance), should have constant horizontal velocity and it's vertical velocity should have a constant acceleration of -9.80 m/s^2

The Attempt at a Solution

I was thinking air resistance and the weight of the ball, but not sure if that makes sense in terms of giving me a larger acceleration than expected.

 

[haruspex]

What exactly is taking the measurements? Do you have a way to calibrate it?
Can you post your y, t data in a machine-readable form (i.e. not as a mere image)?