One way of coming up with a rough estimate is to talk to someone who does have experience with the type of task you're facing. (The folks at Home Depot don't know me and they've never been in my basement, but they're surprisingly good at estimating how long it will take me to paint it, if I give them just a little bit of information.) If no one in your organization has any experience, you might need to hire a consultant.
The margin of error is going to depend on your best-case and worst-case estimates. The text we're using in my Systems Analysis course gives the following formula (Three-point estimation) for calculating most likely duration:
D = ((1 * OD) + (4 * ED) + (1 * PD)) / 6
Where:
- OD = Optimistic Duration
- ED = Expected Duration (your best guess)
- PD = Pessimistic Duration
- D = Most likely duration
Source: Systems Analysis and Design Methods
The formula is simply a weighted average of your own estimates, so it's really only good for turning the range [worst case - best case] into a single numerical estimate. If your input estimates are only guesses then the output will be no better. The important thing to take away from the formula is that your margin of error can be very wide depending on your PD and OD values.
