An understanding of the ZOPA is critical for a successful negotiation, but the negotiants must first know their BATNA (best alternative to a negotiated agreement), or "walk away positions". To determine whether there is a ZOPA both parties must explore each other's interests and values. This should be done early in the negotiation and be adjusted as more information is learned. Essential is also the ZOPA’s size. Where a broad ZOPA is given, the parties might use strategies and tactics to influence the distribution within the ZOPA. Where the parties have a small ZOPA, the difficulty lies in finding agreeable terms.