The period is the number of oscillations (or swings).
To reduce error you can measure 10 swings instead of one and then divide the time taken by 10. As this immensely reduces the error of reaction time!!
hope this helps ^.^
Pendulum is an object that, due to action of gravitational force, swings back and forth at a regular rate around a fixed point if it is pulled aside and let go. A simple pendulum consists of a weight hanging at the end of a string or wire. The path travelled by the weight is called the arc of the pendulum. The period of oscillation is the time it takes the weight to pass back and forth once over this arc. The length of the pendulum is the distance between the point at which the pendulum is fixed, called pivot, and the center of gravity of the weight at the other end of the string or wire of the pendulum.
The period of oscillation of a pendulum depends on the gravitational pull acting on the pendulum and the length op pendulum.
To eliminate error in measuring the period, it is better to time multiple swings, say 10 or 20, and then calculate the average period of 1 swings. Also first few swings after setting the pendulum in motion should not be timed. Also timing and counting of number of swings should be started when the pendulum weight is at one of its extreme points.
The period of the pendulum is the time taken for the pendulum to make one complete to and fro motion called oscillation. The formula for obtaining the Period T of the pedulum whose length is L is given by: T = 2Pi sqrt(L/g).
Caution: In a simple pendulum, the motion is simple harmonic for small amplitude (or angles). So, care should be taken to have small oscillations. The length of the pendulum should be considered entire thread till the centre of mass of the bob( including the radius of the bob if it is spherical). Though the period is not dependent on the weight of the mass of the pendulum, the weight of the bob must keep the thread straight without slackness. It is better the thread is thin weightless and sufficiently long.