To help make digital data more reliable when moving through noisy channels, 'error correction' methods are used. These methods allow one or more errors to be detected and possibly fixed at the receiving end.
'Noisy' means interference. For instance, electrical interference in a data cable or lightning storms affecting radio channels.
One such method or error detection is called 'parity checking'. This involves adding an extra bit to each data packet. When an error is detected, a 'parity error' is indicated in some way.
For example, hard disks make use of parity checking, as does RAM. So do DVD players, Mobile Phones, Fibre-Optics, Radio broadcasts, Digital Television and many other data communicating technologies.
There are two types of parity error checking - Even and Odd
Even parity is checking that the number of 1's in the data word adds up to an even number
Odd parity is checking that the number of 1's in the data word adds up to an odd number.
'Even Parity' checking can only detect an odd number of bits being corrupted (one, three etc) If the errors have caused an even number of bits to change, then parity checking cannot detect the problem.
This is why more complicated error checking schemes have been developed. But parity checks are still very common as it is such a simple way of detecting errors.
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Click on this link: Parity check