Complex sentence: Simple averaging was found to be ineffective, and statistical methods were later used: operators were later asked to grade the reliability of their readings so that poor and variable ones were given less weight than those that appeared stable and well-defined.
Split into simpler sentences: Simple averaging was found to be ineffective, and statistical methods were later used. Operators were also asked to grade the reliability of their readings so that poor and variable ones were given less weight than those that appeared stable and well-defined.