NORMAL FORM GAMES AND DOMINANCE
Hello, dear friend, you can consult us at any time if you have any questions, add WeChat: THEend8_
LECTURE 2: NORMAL
FORM GAMES AND
DOMINANCE
1
LAST WEEK
• Decisions under certainty: completeness and transitivity mean that preferences can
be represented by utility numbers.
• Decision under uncertainty: we can select dominant options and eliminate dominated
ones
2
THE IMPORTANCE OF FIXING THE DOMINATING
CHOICE
• Is there any dominated choice?
No! Neither Beach nor Park is strictly better than Mall in all states.
SUNNY RAINY
Beach 3 0
Park 0 3
Mall 2 2
A SLIGHTLY DIFFERENT BEACH-LOVING HIPSTER
• Is there a strictly dominated choice for this hipster?
No! Beach is not as it is the best when Sunny; Mall is not as it is the best when Rainy; Park is not as
none of the other two choices is always strictly better
• But you may feel that I am just being too stringent: Although Beach is not strictly better than
Park all the time, Beach is always at least as good as Park, and sometimes strictly better.
SUNNY CLOUDY RAINY
Beach 9 8 4
Park 7 8 4
Mall 3 3 8
WEAKLY DOMINATED CHOICE
• Note 1: The dominating choice c’ has to be no worse than c in each and every state –
being better for just one state does not qualify
• Note 2: It is important to fix the dominating choice c’ – you cannot use different
dominating choices for different states
Definition
A choice, say choice c, is Weakly Dominated by
another choice c’ if and only if:
1. c’ is no worse than c in each and every state; and
2. there is at least one state in which c’ is strictly
better than c.
SO FAR…
• We have not used probability!
• Also, we have only made comparisons between choices, the magnitude of the
differences in utility numbers across choices does not matter
• In other words, the concept of dominance (dominant choice, dominated choice) does
not require:
Probability sophistication
Information on the magnitude of the payoffs
BUT DOMINANCE CAN ONLY TAKE US SO FAR
• Is there any (strictly or weakly) dominated choice?
No.
Beach is strictly the best when Sunny
Park is strictly the best when Cloudy
Mall is strictly the best when Rainy
SUNNY CLOUDY RAINY
Beach 10 7 0
Park 3 8 1
Mall 4 4 9
BUT DOMINANCE CAN ONLY TAKE US SO FAR
• If it is likely to be Sunny, we would expect Beach
• If it is likely to be Cloudy, we would expect Park
• If it is likely to be Rainy, we would expect Mall
• When we start using words like “likely”, we are starting to talk about probability
SUNNY CLOUDY RAINY
Beach 10 7 0
Park 3 8 1
Mall 4 4 9
PROBABILITY AND MAGNITUDE
• Even if the probability of Listeria contamination is very small, given the dire
consequence (versus the small improvement in utility when Safe), it seems
reasonable to choose Avoid
• This suggests that we cannot take utility to be ordinal anymore.
Safe Listeria Present
Eat soft cheese 2 – 1,000
Avoid soft cheese 0 0
PROBABILITY AND UTILITY
• Perhaps we can take the weighted average of the utility numbers (weighted by the
probabilities) for each choice
• Then we will pick (or predict) the choice with the highest weighted average
• This is known as Expected Utility
Sunny Cloudy Rainy
Probability 0.4 0.3 0.3 Expected Utility
Beach 10 7 0
Park 3 8 1
Mall 4 4 9
EXPECTED UTILITY: WHAT DOES IT MEAN?
• Expected utility is easy to use (this is a great advantage!)
• But what are we actually assuming when we think that a decision-maker will pick the
choice with the highest expected utility?
Sunny Cloudy Rainy
Probability 0.4 0.3 0.3
Beach Fun Breezy Wet
Park Hot Nice Soggy
Mall Meh Meh Comfy
A BIT OF TERMINOLOGY
• Given the probabilities on the states of nature, a choice leads to a probability
distribution over outcomes
• In our example, Beach leads to the following probability distribution:
• We call these distributions lotteries
Fun Breezy Wet Hot Nice Soggy Meh Comfy
Prob 0.4 0.3 0.3 0 0 0 0 0
LOTTERIES: GENERAL CASE
• In general, if there are n possible state-contingent (i.e., certain) outcomes
1, 2, … , , then a lottery can be described by:
• …where the probabilities sum up to 1.
• What economists typically do is to start with the preliminaries of how a decision-
maker compares between different lotteries
• Such comparisons are called Preferences over Lotteries
…
Prob 1 2 …
EXPECTED UTILITY PROPERTY
• Utility functions that have the expected utility property are often called von-Neumann
Morgenstern (vNM) utility functions or Bernoulli utility functions
Definition
A function V that assigns numbers to every lottery has the expected utility
property if:
= 1 1 + 2 2 + ⋯+ = �
=1
()
Where L induces a lottery defined by the probabilities p and the payoffs x.
MAGNITUDE MATTERS FOR EXPECTED UTILITY
• The expected utility property requires:
(L) = �
=1
()
• But once I started multiplying and adding, the differences between the numbers have
meanings
• The utility numbers () carry more content than mere comparisons
• In technical terms, we say that the utility . is not ordinal, but cardinal
Go to www.menti.com and enter the code:
3268 8499
1 question
WHY WOULD PEOPLE USE EXPECTED UTILITY?
• Let’s denote lotteries as , , , …
• The lottery that puts probability 1 on outcome will be written simply as
• ≽ read as is weakly preferred to
ASSUMPTIONS ON PREFERENCES
• One questions we can ask:
• “What do we need to assume about the preference over lotteries, in order for us to
use expected utility?”
• These assumptions are sometimes known as axioms
They are principles of behaviour
If we abide by them, then we use expected utility to choose between lotteries
AXIOMS ON PREFERENCES OVER LOTTERIES