Their effect cannot be reduced by repetition of observations. Where possible they are eliminated by observational procedures, e.g. by taking FL and FR readings with a theodolite. Periodic and Random Errors are of varying sign. Their effect is therefore reduced by taking the mean of repeated observations. No matter how large they are in each observation, the mean can be made more precise by taking more observations.
Small errors occur frequently and are therefore more probable than large ones.
Positive and negative errors of the same size are equally probable and happen with equal frequency.