How do you handle incomplete information in negotiations?

Information is often if not always incomplete, its either lack of time time to develop the terms of the engagement or natural evolution of a project. In simple terms it will come down to your apporach to risk as the incomplete element cuts both ways. Both Parties are in the same boat. I have worked on negotiations where both parties have agreed that there are elements that require more detail and it has been agreed to work on these to get to a point where both are content. There is a premise in the question that suggests a time imperative , however as in all negotiaitons if you arent happy or comfortable walk away

Preparation is key - knowing the answer to this question BEFORE coming to the negotiation table saves you a lot of headache in the future. Once you know what you don't know, you can work to complete your understanding before beginning negotiations.

I agree, having a full picture of where you stand, including where some information is missing is paramount to having a successful negotiation.

Incomplete information whatever the reasons at best leads to ambiguity and unnecessary effort even when identified. Caused by a lack of attention / time & or will at project intent outset, this is generic in the fit-out industry (joinery). To this end as an ex manufacturer. I've set up a consultancy business to assist in the development of the detail & the compilation of the scope, enabling all prospective tenders to have the same & maximum information available & to clearly set out areas of missing info, stipulating a PC sum to be included at bid stage. It is completely unacceptable to expect a sub contractor to "make a guess" against others & time, & must be the responsibility of the client team to supply correct data....utopia perhaps...

In my experience I find it very useful to choose 2-3 options within the possible paths of the negotiation, access the risks and opportunities on each one and define boundaries of where I am willing to go.

Incomplete information is a given in almost all negotiations. That shouldn’t stop you - or both parties, really - from gathering as much as you can as you go along. And during that time, build trust between the parties because that trust will lead to more information sharing. With more on the table, there’s more opportunity to create value, not just take from one side or the other. It’s exceedingly rare to have nothing else to give, or trade, or share, as part of a multifaceted negotiation. The more both parties know about what matters to the other side, the more likely a third space emerges where more value can be created. This won’t happen in a low trust environment, so use the time wisely to build rapport and reinforce positive behavior.

I find that for the most part creating empathy with the other party is often useful to create trust and encourage information sharing. This is much easier to do face to face than through phone conversations or even video calls so I try to schedule a presencial meeting if at the first available opportunity. The goal on this meeting should be first of all to know the other side and should avoid the more sensitive subjects if possible