Fake Data Example
To quickly show it works, we can create fake data. Here, we generate 1000 repetitions of a ten-round game in which each player starts by making a random move, and in subsequent rounds, one player follows a “tit-for-tat” strategy while the other one follows a “noisy tit-for-tat” strategy that’s equivalent to tit-for-tat, except that with a 10% probability the player will make a random move.
seed <- 1900
set.seed(seed)
cdata <- data.frame(outcome = NA,
period = rep(1:10, 2000),
my.decision1 = NA,
other.decision1 = NA)
#
# Prisoner's dilemma
#
pd_outcome <- function(player_1, player_2) {
#
# 1 = C
# 2 = D
#
player_1 + 1
}
tit_for_tat <- function(last_round_self, last_round_opponent) {
last_round_opponent
}
noisy_tit_for_tat <- function(last_round_self, last_round_opponent) {
if (runif(1,0,1) <= 0.1) {
sample(0:1,1)
} else {
last_round_opponent
}
}
for (i in seq_along(cdata$period)) {
if (cdata$period[i] == 1) {
my.decision <- sample(0:1,1, prob = c(0.9,0.1))
other.decision <- sample(0:1,1)
cdata[i, "outcome"] <- pd_outcome(my.decision, other.decision)
} else{
my.last <- my.decision
other.last <- other.decision
my.decision <- tit_for_tat(my.last, other.last)
other.decision <- noisy_tit_for_tat(other.last, my.last)
cdata[i,c("outcome", "my.decision1", "other.decision1")] <- c(pd_outcome(my.decision, other.decision), my.last, other.last)
}
}The only required argument of the main function of the package, evolve_model, is a data.frame object, which must have 3-5 columns. The first two columns must be named “period” and “outcome” (period is the time period that the outcome action was taken). The remaining one to three columns are predictors, and may have arbitrary names. Each row of the data.frame is an observational unit, an action taken at a particular time and any relevant variables for that time. All of the (3-5 columns) should be named. The period and outcome columns should be integer vectors — e.g. c(1,2,1) — and the columns with the predictor variable data should be logical vectors — e.g. c(TRUE, FALSE, FALSE) — or vectors that can be coerced to logical with as.logical().
Here are the first eleven rows of this fake data:
| outcome | period | my.decision1 | other.decision1 |
|---|---|---|---|
| 1 | 1 | NA | NA |
| 1 | 2 | 0 | 0 |
| 1 | 3 | 0 | 0 |
| 1 | 4 | 0 | 0 |
| 1 | 5 | 0 | 0 |
| 1 | 6 | 0 | 0 |
| 1 | 7 | 0 | 0 |
| 1 | 8 | 0 | 0 |
| 1 | 9 | 0 | 0 |
| 1 | 10 | 0 | 0 |
| 1 | 1 | NA | NA |
We can estimate a model with that data. evolve_model uses a genetic algorithm to estimate a finite-state machine (FSM) model, primarily for understanding and predicting decision-making. This is the main function of the datafsm package. It relies on the GA package for genetic algorithm optimization. We chose to use a GA because GAs perform well in rugged search spaces to solve integer optimization problems, are a natural complement to our binary representation of FSMs, and are easily parallelized.
evolve_model takes data on predictors and data on the outcome and automatically creates a fitness function that takes training data, an action vector that evolve_model generates, and a state matrix evolve_model generates as input and returns numeric vector whose length is the number of rows in the training data. evolve_model then computes a fitness score for that potential solution FSM by comparing it to the provided outcome in the real training data. This is repeated for every FSM in the population and then the probability of selection for the next generation is proportional to the fitness scores. If the argument cv is set to TRUE the function will call itself recursively while varying the number of states inside a cross-validation loop in order to estimate the optimal number of states, then it will set the number of states to that optimal number and estimate the model on the full training set.
If the argument parallel is set to TRUE, then these evaluations are distributed across the available processors of the computer using the doParallel functions; otherwise, the evaluations of fitness are conducted sequentially. Because this fitness function that evolve_model creates must loop through all the training data every time it is evaluated and we need to evaluate many possible solution FSMs, the fitness function is implemented in C++ to improve its performance.
res <- evolve_model(cdata, seed = seed)evolve_model evolves the models on training data and then, if a test set is provided, uses the best solution to make predictions on test data. Finally, the function returns the GA object and the decoded version of the best string in the population. Formally, the function return an S4 object with slots for:
callLanguage from the call of the function.actionsNumeric vector with the number of actions.statesNumeric vector with the number of states.GAS4 object created byga()from theGApackage.state_matNumeric matrix with rows as states and columns as predictors.action_vecNumeric vector indicating what action to take for each state.predictiveNumeric vector of length one with test data accuracy if test data was supplied; otherwise, a character vector with a message that the user should provide test data for better estimate of generalizable performance.varImpNumeric vector with length equal to the number of columns in the state matrix, containing relative importance scores for each predictor.timingNumeric vector length one containing the total elapsed time it tookevolve_modelto execute.diagnosticsCharacter vector length one, designed to be printed withbase::cat().
Use the summary and plot methods on the output of the evolve_model() function:
summary(res)## Length Class Mode
## 1 ga_fsm S4plot(res, action_label = ifelse(action_vec(res)==1, "C", "D"),
transition_label = c('cc','dc','cd','dd'))
Result of plot() method call on ga_fsm object.
The diagram shows that evolve_model recovered a tit-for-tat model in which the player in question (“me”) mimics the last action of the opponent.
Use the estimation_details method on the output of the evolve_model() function:
suppressMessages(library(GA))
plot(estimation_details(res))
Result of plot() method call on ga object, which is obtained by calling estimation_details() on ga_fsm object.