How we change what others think, feel, believe and do
When two events can occur separately or together, the conjunction, where they overlap, cannot be more likely than the likelihood of either of the two individual events. However, people forget this and ascribe a higher likelihood to combination events, erroneously associating quantity of events with quantity of probability.
Kahneman and Tversky offered the following problem to a number of people:
Linda is 31, single, outspoken and very bright. She majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice and also participated in antinuclear demonstrations.
They then asked whether she was more likely to be (a) a bank teller, or (b) a bank teller and active in the feminist movement. 86% answered (b).
If someone says an out-of-stock product in a shop may be in within the next two weeks and it may be in this week, and it may be in tomorrow, then it seems more likely it will be in sooner rather than later.
When persuading about something that is not guaranteed every time, show how it appears in several different scenarios.
Remember your mathematics! Just because something can happen in different circumstances it does not make it more likely.