define_model function

Define and evaluate model expression

A model expression is defined by specifying random number generation functions for a probabilistic sensitivity analysis (PSA) and transformations of the sampled parameters as a function of input_data. The unevaluated expressions are evaluated with eval_model() and used to generate the model inputs needed to create an economic model.

define_model(tparams_def, rng_def, params = NULL, n_states = NULL)

eval_model(x, input_data)

Arguments

• tparams_def: A tparams_def object or a list of tparams_def objects. A list might be considered if time intervals specified with the times argument in define_tparams() vary across parameters. Parameters for a transition probability matrix (tpmatrix), utilities (utility), and/or cost categories (costs) are returned as a named list (see define_tparams()

  for more details).

• rng_def: A rng_def object used to randomly draw samples of the parameters from suitable probability distributions.

• params: Either (i) a list containing the values of parameters for random number generation or (ii) parameter samples that have already been randomly generated using eval_rng(). In case (ii), rng_def should be NULL.

• n_states: The number of health states (inclusive of all health states including the the death state) in the model. If tpmatrix is an element returned by tparams_def, then it will be equal to the number of states in the transition probability matrix; otherwise it must be specified as an argument.

• x: An object of class model_def created with define_model().

• input_data: An object of class expanded_hesim_data

  expanded by patients and treatment strategies.

Returns

define_model() returns an object of class model_def, which is a list containing the arguments to the function. eval_model() returns a list containing ID variables identifying parameter samples, treatment strategies, patient cohorts, and time intervals; the values of parameters of the transition probability matrix, utilities, and/or cost categories; the number of health states; and the number of random number generation samples for the PSA.

Details

eval_model() evaluates the expressions in an object of class model_def returned by define_model() and is, in turn, used within functions that instantiate economic models (e.g., create_CohortDtstm()). The direct output of eval_model() can also be useful for understanding and debugging model definitions, but it is not used directly for simulation.

Economic models are constructed as a function of input data and parameters:

1. Input data: Objects of class expanded_hesim_data

   consisting of the treatment strategies and patient population.

2. Parameters: The underlying parameter estimates from the literature are first stored in a list (params argument). Random number generation is then used to sample the parameters from suitable probability distributions for the PSA (rng_def argument). Finally, the sampled parameters are transformed as a function of the input data into values (e.g., elements of a transition probability matrix) used for the simulation (tparams_def argument). The params argument can be omitted if the underlying parameters values are defined inside a define_rng() block.

Examples

# Data
library("data.table")
strategies <- data.table(strategy_id = 1:2,
                         strategy_name = c("Monotherapy", "Combination therapy"))
patients <- data.table(patient_id = 1)
hesim_dat <- hesim_data(strategies = strategies,
                       patients = patients)
data <- expand(hesim_dat)

# Model parameters
rng_def <- define_rng({
  alpha <- matrix(c(1251, 350, 116, 17,
                    0, 731, 512, 15,
                    0, 0, 1312, 437,
                    0, 0, 0, 469),
                  nrow = 4, byrow = TRUE)
  rownames(alpha) <- colnames(alpha) <- c("A", "B", "C", "D")
  lrr_mean <- log(.509)
  lrr_se <- (log(.710) - log(.365))/(2 * qnorm(.975))
  
  list(
    p_mono = dirichlet_rng(alpha),
    rr_comb = lognormal_rng(lrr_mean, lrr_se),
    u = 1,
    c_zido = 2278,
    c_lam = 2086.50,
    c_med = gamma_rng(mean = c(A = 2756, B = 3052, C = 9007),
                      sd = c(A = 2756, B = 3052, C = 9007))
  )
}, n = 2)

tparams_def <- define_tparams({
  rr = ifelse(strategy_name == "Monotherapy", 1, rr_comb)
  list(
    tpmatrix = tpmatrix(
      C, p_mono$A_B * rr, p_mono$A_C * rr, p_mono$A_D * rr,
      0, C, p_mono$B_C * rr, p_mono$B_D * rr,
      0, 0, C, p_mono$C_D * rr,
      0, 0, 0, 1),
    utility = u,
    costs = list(
      drug = ifelse(strategy_name == "Monotherapy",
                    c_zido, c_zido + c_lam),
      medical = c_med
    ) 
  )
})

# Simulation
## Define the economic model
model_def <- define_model(
  tparams_def = tparams_def,
  rng_def = rng_def)

### Evaluate the model expression to generate model inputs
### This can be useful for understanding the output of a model expression
eval_model(model_def, data)

## Create an economic model with a factory function
econmod <- create_CohortDtstm(model_def, data)

See Also

define_tparams(), define_rng()