The error due to known causes is known as systematic. Systematic error is of same sign i.e. if due to some cause, the error is of positive sign then in all the measurements the sign of error will be positive and if due to some cause, the error is negative sign then in all the measurements the sign of error will be negative.
Types of systematic errors:
1. Instrumental error
2. Personal error
3. Error due to external causes
4. Error due to imperfection
Random error : When an experiment is repeated, it is found that all the measured values are slightly different from each other. There is no definite pattern in the variation of measured values. The difference between true value and measured value (known as absolute error) does not have any methodical pattern of sign and magnitude, but it is random. This type of error is called random error.