Answer
A random error has equal probability of being skewed in either direction (hence random). A systematic error will always lean in one direction, either consistently too high or consistently too low.
Work Step by Step
Random Error: Typically, all measurements are considered to have some degree of random error. This is why sample size is important: if there are many trials, then the measurements will essentially average out.
Systematic Error: Systematic error doesn't average out through repeated experiments or trials. If there isn't a balance, then the measurements may systematically read higher than normal or lower than normal.
