How we change what others think, feel, believe and do
In negotiation, one of the biggest traps is to think in absolutes, that you will succeed or win, that you want A and not B, that the other person is good or bad. When an exchange is being hammered out, the more things you can change, the more options you have and the greater the chance of finding an acceptable exchange.
The secret of this in in finding and working with the variables.
Continuous and discrete
Variables can be continuous, such as temperature or change in discrete steps, such as people. Discrete steps can be large, such as houses or small, such as with money. You can find continuous and discrete variables by asking 'How much?' or 'How many?'.
Units of measurement
Variables are those things that can be changed along multiple dimensions, each of which may have a different unit of measurement.
For example, when talking about with price, the underlying unit of measurement is money. Other units of measurement can be added, with variables such as the time taken to pay, how it is paid (cash or other means) and so on.
To identify key negotiation variables, ask 'What are the things that are important to me here?' For example, when buying a car you might include color, price, mileage, cleanliness, damage and so on. You can look at what is being negotiated and ask generally what can vary about these things. For example, the car may have variables in the number of owners, the wear on the tires, the completeness of service history and so on. Look also for variable that are not as obvious, such as the prestige of the car and the likelihood of it being stolen or damaged when on your driveway.
Variables can include:
It is useful then to prioritize variables -- for example how important is damage to you? Is a small scratch acceptable? Also consider how these will affect other variables, for example if there is crash damage, how would that affect your valuation of the car?
The other person's variables
Remember that the other person also has variables, that they may or may not have identified and prioritized. If they have not, then you may be able to facilitate this process during the negotiation to your (and maybe their) advantage. Include emotional factors as well as more substantive ones, for example consider the attachment that the person has to the item in question, their need to sell it (have they already bought a replacement?), their need for money (do they appear affluent) and so on.
And the big