I always consider four factors in estimating software cost

- the scope of the task

- the resource/people you have

- the confidence in the resource/people

- and the dependency on both the resource and the task

Estimation goes wrong in many ways. In my experience, I categorize as follow:

- estimated by non-technical PM, unable to gauge technical/business complexity

- estimated too early in development life cycle. Often time someone come up with idea X then ask right away "shouldn't be difficult, how long will it take"

- no view on team member's productivity due to lack of measurement or measuring the wrong thing

- not considering dependency in the development cycle. e.g. The question "whether our only backend engineering available for the task" is often omitted

- not considering testing, documentation, 3rd party integration/procurement, maintenance and deployment cost

The common practise of coming up a number, then double, or maybe triple it up to make the estimation does not address why estimation is off. Mis-estimation is a symptom. The cause and the cure lies in the people