the obtained number is smaller in magnitude that the accepted value of g. This makes sense because we have to take air resistance into account when we let a ball fall. The air will slow the ball down and will cause us to have a lower value. Another possibility is that this experiment was done at high altitude where g is less than the 9.8 meters per square second which is valid only at sea-level. Still another source of error could be the apparatus we used to do the experiment. The height may not have been measured accurately, or there could be some problem with timing device, but these are both controllable and checkable.