If you are writing a self assessment as part of your performance evaluation at work, you will write it differently than you would for personal reasons.

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Begin by writing your perspective of the objective for your current position. Do not simply copy and paste the objectives from the job description provided by your employer or use a thesaurus to mix up the words a bit. Rather, from your personal point of view, list what you believe the goals and purposes are for your current position. This will help your supervisor determine whether you have a clear understanding of what is expected of you. This can also clue your superiors in if you are picking up slack for another employee or department, which could mean a pay increase, promotion or more assistance for you.
List any noteworthy achievements you have made since your last review or assessment. If you think that you have not accomplished anything significant, consider your daily habits and you are sure to come up with something. You might have saved the business money by recycling copy paper, for example, or perhaps you increased the company's visibility by proposing the sponsorship of a local school.
Write down reasons that the business is faring better because of the position you hold. You do not have to sound arrogant or boastful. Simply list factual benefits that you've offered. This might be the way you mentored an intern or how, as a member of the support staff, you meet the needs of others in a timely manner.
Talk about the goals you set in your last evaluation. Explain what steps you have taken to meet those goals and how you have grown professionally during the process.
Write about hurdles you have had to overcome since your last review and how you did so. Discuss the type of support you received from supervisors and colleagues, as well as what might have helped you even more.
Finally, round out the self assessment by including goals for the next period. Make sure your goals are measurable. This means your goals should state specifically what you want to accomplish, how you will do so, and by when. Rather than stating, "I will learn to type faster," a measurable goal might be, "I will increase my typing speed to 75 wpm by the end of June using typing software."
By providing a thorough, well thought out self assessment, you can show your superiors what you have to offer as well as what you hope to gain.