Here are my key takeaways from the book. I’ve found that the earlier chapters were more interesting to me as there were more statistical concepts at the start. There are many game theory models in the later chapters:
-Many-model approach: The book relies on a many-model approach. A given problem can usually be solved with several models. Also, different problems require different models. (pp. 5-6) Condorcet jury theorem: Many models are better than one model in a jury problem. If a judge is right more often than not, a decision will be more accurate if there are multiple jurors. (p. 28) Diversity prediction theorem: Many-model error = Average-model error - diversity of model predictions. (pp. 29-30)
-The wisdom hierarchy: Data, information, knowledge, wisdom. Data is just raw data. Information is categorical data. For example, region-labeled temperature data is information. Knowledge is “justified true belief” (Plato) and contains additional relational information such as correlations, causation, logic, and conditions for the models to hold. Knowledge is organized information. Models are knowledge. Wisdom is the “ability to identify and apply relevant knowledge”. (pp. 7-8)
-Models can be divided into embodiment models (such as a discounted cash flow model), analogous models (such as a valuation multiple model), and alternative reality models (such as the game of life; they model hypothetical scenarios and not an actual process). (pp. 13-14)
-The seven uses of models: REDCAPE (p. 15):
--Reason (identify conditions and deduce logical implications);
--explain;
--design processes, institutions, policies, and rules;
--communicate: Clear sets of inputs and outputs and corresponding definitions of all inputs and outputs that can be agreed upon. (pp. 20-21)
--act;
--predict;
--explore.
-Bagging means “bootstrap aggregation” and constructs many models. Many datasets are drawn and models are fitted. The average of the models is the final model. (p. 42)
-Rule-based models work without loss functions. (p. 53)
-Models produce an equilibrium, cycles, randomness, or complexity. (p. 56)
-Rational-choice models and zero-intelligence models provide upper and lower bounds for potential outcomes. (p. 58)
-Multiplication of random variables leads to lognormal distributions. Addition of random variables leads to normal distributions. (p. 60) “Lognormal distributions lack symmetry because products of numbers larger than 1 grow faster than sums ... and multiples of numbers less than 1 decrease faster than .. sums. If we multiply sets of twenty random variables with values uniformly distributed between zero and 10, their product will consist of many outcomes near zero and some large outcomes, creating the skewed distribution shown in 5.2.” (p. 66) Long-tailed distributions require non-independence. Those often come in the form of positive feedback. Sales, fires, city populations. (p. 69) Power-law distributions have many small events. (p. 70) Power laws with an exponent of 2 or less lack a well-defined mean. (p. 71)
-Sampling random variables: \sigma_\mu = \frac{\sigma}{\sqrt{N}} and \sigma_{\sum} = \sigma\sqrt{N} (p. 63)
-Zipf’s law: “The special case of power laws with exponents equal to 2 are known as Zipf distributions. For power laws with exponents of two, an event’s rank times its probability will equal a constant, a regularity known as Zipf’s law. Words satisfy Zipf’s law. The most common English word, the, occurs 7% of the time. The second most common word, of, occurs 3.5% of the time. Notice that its rank, 2, times its frequency of 3.5% equals 7%.” Event Rank * Event Size = Constant (p. 72)
-Self-organized criticality, forest fire model: Self-organized criticality leads to power-law distributions for the sizes of forest fires. “The key assumptions for self-organization to critical states is that pressure increases smoothly, like water flowing into the lake, and that pressure decreases in bursts, including possibly large events. (pp. 74-75)
-Projects with large budgets over run out of control. This is because the random variables, i.e., the budgets for individual stages and parts of a project, are positively dependent on each other. If one part encounters problems, the next step will also run into budgetary problems. (pp. 78-79)
-Models of value and power: LOTB and Shapley values. LOTB is the last-on-the-bus value. How much would a participant add to the system if he joined last? Shapley values are the average value of a participant for each possible condition under which the participant could enter the system. (p. 107)
-In a network model, a “node’s betweenness score equals the percentage of minimal paths that go through a node.” (p. 119)
-Friendship paradox: “One analysis of friendships on Facebook found that the average person has around two hundred friends and their friends, on average, have more than six hundred friends.” (p. 124)
-Network robustness. “Here, we consider the question of how the size of the largest connected component of the network, the giant component, changes as nodes randomly fail… In the random network, the size of the giant component falls linearly at first. At a critical value where the probability of an edge equals 1 divided by the number of nodes, the size of the largest component falls to an arbitrarily small proportion of the original network size. The small-world network shows no such abrupt change. A majority of t connections exist within the geographic clusters.” (p. 127)
-”Most consumer goods and information spread through both diffusion and broadcast. Our next model, the bass model, combines the two processes in a single model.” (p. 136)
-SIR model: The susceptible-infected-recovered model has a built-in saturation assumption. (p. 137)
-The minimum percentage of immunity in a population, the vaccination threshold, depends on the basic reproduction number (R_0), which says how many people get infected by each person that is newly infected. The vaccination threshold is: V \geq \frac{R_0 - 1}{R_0}. Polio has a R_0 of 6, so the vaccine must cover ⅚ of the population.(p. 138-139)
-Superspreaders are key to epidemics. This is due to degree squaring. Superspreaders have more contacts to others, which exposes them to more risk for getting a disease and also increases the chances to spread the disease in the same manner. A superspreader with 3x more social contact will therefore be 3x more likely and 3x more dangerous when infected, making him 9x more dangerous overall. (pp. 139-140).
-R_0 and the vaccination threshold are contextual tipping points. Small changes in the context (environment) can change outcomes significantly. The current focus in public discussions around R_0 and vaccinations show that there is awareness for the criticality of these measures. (p. 141)
-Grossman and Stiglitz paradox: If investors believe in the efficient market hypothesis, they stop analyzing, making markets inefficient. (p. 160)
-Agent-based models are computer programs that model each agent individually. (p. 213)
-Hedonic attributes: more is always better. Spatial attributes: sweet spots that are different among agents. (pp. 227-228)
-Voronoi neighborhoods help to see which characteristics a product should have to optimally fit a certain customer group. (p. 231)
-”A one-dimensional model implies that candidates position at the median. Higher-dimensional models imply that they should not. Which type of model do we believe? We should place complete faith in neither model, but instead gain insights from both… We should instead expect complexity, an endless dance of competition for votes through coalition building.” (p. 234)
-Models of cooperation: In this example: a lone cooperator cannot spawn an additional cooperator, but two adjacent cooperators can. It follows that a small cluster of cooperative nodes surrounded by empty cells could expand into open nodes. Therefore, regions of cooperation can emerge from a handful of cooperators. (p. 264) Group selection is another way to achieve efficient cooperation. Let groups form. The best group (the one with many cooperators) will emerge as a winner and make copies of itself or otherwise project its system on the rest of the population. This can be easier than to just have one large population that has to be made cooperative. (p. 265)