Research has shown that people can rarely self-assessment accurately. If the person self-assessing has low self-confidence, than this will be reflected on his/her self assessment. Also there are cultures where self promotion is negatively viewed and this too may lead someone to completing a less than positive self review. Lots of characteristics may impact how one self-assesses: race, gender, beliefs, religion, etc.
On the other side of the coin are individuals raised in competitive environments where self-promotion is not only welcome but encouraged. In these cases an individual may take credit for group work in an attempt to "win points".
If your company forces you to conduct evaluations based on self-rating then it is important you consciously determine your employees tendencies and use that knowledge to erase over/under self-evaluations in an attempt to be fait, objective and manage with integrity.
I have spoken to some organizational researchers and have read hundreds of reports, I can find no objective research that shows that sharing self-assessments before the formal manager-employee review contributes to a better or more accurate evaluation outcome.
On the contrary, there seems to be research showing that these self-evaluations may actually bias the reviewing manager and that any bias adjustments made (if at all) are inadequate to compensate for the actual gap.
Knowing this, I believe these self assessments are a historic relic of days gone by and should be completely abolished as an HR practice. What do you think?