Error Detection or Parity Check Method : A general way to detect errors is to append a special bit to every word. One popular technique employs a single check bit called parity bit. The parity bit is append to an nbit word X = x_{0}, x_{1},....., x_{n1 }to
form the (n+1)bit word X^{*} = x_{0}, x_{1},....., x_{n1},c_{0 .}Here c_{0 }is the parity bit. Consider the following,
Taking the original data to be sent is,
form the (n+1)bit word X^{*} = x_{0}, x_{1},....., x_{n1},c_{0 .}Here c_{0 }is the parity bit. Consider the following,

Appending the parity bit and the changed data is,
Here, we put the c_{0 } as 0 because the word is containing even number of “1”s in it. If the number of “1”s is odd then we will put 1 as the value of c_{0 }.
Now, if any single bit of the data is changed then the parity bit will not match, as the number of “1”s will change. Similarly, if the parity bit is changed then it will not match with the number of “1”s in the data. So the decoder will be able to detect a single bit error.
This technique will fail when multiple bits will change.