Why can estimates not be accurate?

Estimating software development effort is not just a well-established routine anymore. I'm afraid that it is evolving into a branch of science! For decades now, software teams have tried to come up with various methods to predict timelines.

About 15 years ago, I used Function Points as a way to assess the size of an application, based on how much input / output data the application had to handle. This was at a stage where a system view of a software was more prevalent and the applications were mostly about data flow. There was little user input. User experience was never discussed. This meant that software developers could be given requirement specifications, with details of what field to input and what to validate ...

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