By 
What are they?
A pedometer is a device the size of a buzzer that is attached to your waist and counts the number of steps you take.
Are they accurate?
Pedometers detect the vertical movement of your hips as you walk. Studies show that a pedometer is more accurate when a person walks faster than 3 mph. As speed decreases, so does precision and this is due to the mechanics of walking at a slower speed. People tend to drag or drag their feet at slower speeds and this results in less vertical movement of the hips than the pedometer needs to count steps. Accuracy at 3 mph or more can exceed 96% and at 2-3 mph can drop to 74%. Below 2 mph, the accuracy can be as low as 60%. Needless to say, a brisk walk is needed here.
How do pedometers work?
There are 2 types of pedometers in terms of internal workings. The first type is a mechanical spring lever that has a horizontal bar that is suspended by a spring parallel to the ground. When you walk and induce a vertical force or movement in the bar, it moves upward to complete an electrical circuit. Each closing and opening of the circuit is counted as one step.
The second type is also a beam but with a weight at the free end. At the other end, the beam is connected to a piezoelectric cell. When vertical movement is applied, a pressure is induced in the cell that produces a voltage that is proportional to the applied force. These voltage swings are used to count your steps.
Which type is better?
Piezo type pedometers are more sensitive to slower speeds and tend to be more accurate below 2 mph. They are also not as sensitive to tipping over from their horizontal position as a spring lever type is. A piezo-type pedometer can only see a 10% reduction in accuracy when tilted at speeds below 2 mph, while a spring lever can see a reduction in accuracy of up to 60% below 2 mph. So if you’re older or overweight and walk slowly, piezoelectric design is probably the way to go.
Pedometers should be worn at the waist in line with the leg. If you have trouble fastening it at the waist, you can fasten it to your sock. This should give you the same necessary vertical movement.
Accuracy test of your pedometer
When you first get your new pedometer and want to check its accuracy or want to do interim checks on one you own now, set the pedometer to zero. Count 100 steps carefully to see what kind of result you get. Remember you are looking for an accuracy of +/- 10%, which means anything in the range of 90-110 steps. If you don’t get it in this range, try repositioning your pedometer and walking again to see if that makes a difference. Some people have different gaits and it is just a matter of trial and error to find the best position for you.