I’ve noticed that even people who apparently know what they’re talking about use the words “estimate and “deadline” interchangeably. They are not the same thing at all.

An estimate takes the form: ‘I expect task X will be complete at time T.’ An estimate for task X is the expected value for the time at which task X will be completed. Equivalently, saying ‘my estimate for task X is time T’ means that you think it is 50% likely that task X will be completed before time T (and 50% likely that it will be completed after time T). For example, if I say ‘I estimate that the megatron will be ready to go live on August 1st’, I am saying that, ‘to the best of my knowledge, it is equally likely that the megatron will be ready before August 1st as it is that it will by ready after August 1st.’

A deadline takes the form: ‘If task X is completed by time T, do A, else, do B.’ For example, ‘If the megatron is ready by August 1st, start work on the hypertron, else, cancel the expo’.

Notice that deadlines and estimates have different arities. An estimate for a task consists of just a single date-time, but a deadline for a task consists of three things: a date-time, an if-case, and an else-case. It is extremely common to elide the latter two and say ‘the deadline for task X is time T.’ To do so with no context is close to meaningless; usually, it is taken to mean that A=‘do nothing’, and B=‘punish someone’, where ‘someone’ is sinisterly ambiguous.

There is no intrinsic relationship between an estimate for a task X and the time part of a deadline for the task X.

There is also no single estimate for a task X: different people provide different expected values. Nor must there be a single deadline for a task X: it can have none, or many.

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