Importantly is the angle at which the pendulum is drawn out to. The smaller the angle, the closer to harmonic motion it will behave because the restoring force is acting tangent to the arc the pendulum moves through instead of being outside of it. therefore, keep the angle at which you draw out the pendulum less that about 12 degrees.
also, the period of the pendulum is based on the length of it so make sure that the string is one that does not stretch. At the bottom of the swing, the string can stretch affecting it's length and therefore affecting its period. the tension in the string can be calculated T = mg + Fc where Fc is centripetal force. The stretch needs to be eliminated by using a high test fishing line or other, non elastic material.
A third factor of course friction. Use materials that do not have much friction like the fishing line.
finally the shape of the bob is important. the pendulum's mass is measured to the center of mass of the bob. A small, spherical bob is much preferred over a long one. Small lead fishing weights that are spherical are ideal. they also eliminate or minimize a lot of error due to air resistance.
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