I think you are confused by the word "interest". In this case, it means your ownership of the plan and the valuation of the plan. So, if you have a 401(k) with $23,000 in it, the answer is $23,000 (unless you have a loan outstanding from the plan). It is NOT using the word "interest" to mean the earnings on the plan assets, as in "it earned 2.75% interest during the year."
