The problem situation
Every project manager has been there: you need to estimate some work and you have no clue how long this particular thing might take. It is so new that nobody has done it before; it is completely outside your domain of expertise; or it is so complex that you lose track of your thoughts when trying to think it through.
How not to solve it
You are inclined to just do a best guess, and send it around to your team members for feedback. You may add some background and considerations to defend how you arrived at your guesstimate. You think that the wisdom of the crowd will then kick in and the combined knowledge of your whole team will help you arrive at a reasonable estimate.
Why it does not work like you think it would
When you present people with numbers (any number), a brain bias called achoring kicks in. The number will serve as a reference point for the thinking of your team, and all feedback will be relative to that anchor. Because of group behaviour, you are unlikely to receive the feedback that your estimate is off the mark by orders of magnitatude, even if it is… Instead you will receive only ‘acceptable’ corrections to your initial numbers.
What you should do
There is a simple game, called ‘planning poker’, that solves the problems of this brain bias. Convene a meeting with your team, and have everyone write down their estimate for the work, without showing it to anyone else. Then when all are ready, put the cards on the table simultaneously. Now discuss all the major differences in estimates (orders of magnitude) and have your team members explain how they arrived at their various and different estimates. This will surface all the important information gaps between team members, and discussing them will increase the wisdom of your crowd. As an added benefit, you will have commitment on the final estimate, as the group has understood all relevant background and considerations that have now been exchanged in the discussion.
Tip: you can use planning poker cards instead of having people write down their estimates. Download an app, or get a few decks (more fun!)