Broadly speaking, estimation refers to a thinking or valuing process and estimate to the result of such a process.
An estimate of the time needed is the figure produced by working out how long something will take, whereas an estimation of the time needed is the calculation process that produces that figure. Estimation also has the meaning “judgment or opinion,” which estimate does not have: What, in your estimation, is the cause of the problem? She went down in their estimation when the truth came out.
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