Overview

Given a data file (text file, data.frame) with the correct variable names (see section 1.3) and a model file (see section 1.1 and 2) the steps needed to fit the RT-MPT model (Hartmann et al., 2020; Hartmann & Klauer, 2020; Klauer & Kellen, 2018) to the data is described in Section 1:

1.1 Convert the equation/model file to a rtmpt_model via the to_rtmpt_model() function 1.2 If necessary, specify some restrictions 1.3 Convert the data file to a rtmpt_data format via the to_rtmpt_data() function 1.4 Fit the specified model to the data via the fit_rtmpt() function 1.5 Check convergence of the model parameters and getting a summary

In Section 2, 3, and 4 more information about the equation/model files, the data and additional functions is provided.

1 Detailed explanation of the steps

1.1 Model converter

The equation/model file can be provided by a text file or directly like the following. One might use a syntax that is comparable to the EQN syntax by Heck, Arnold, and Arnold (2018) or Hu (1999) or a syntax that is comparable to the MDL syntax by Singmann and Kellen (2013). In the EQN syntax you can separate trees, categories and paths either by using semicolons (like in the example below) or commas, but not mixed:

eqn = "
# CORE MODEL
## EQN SYNTAX

## tree ; cat ;          mpt
      0 ;   0 ;           Do
      0 ;   0 ;     (1-Do)*g
      0 ;   1 ; (1-Do)*(1-g)

      1 ;   3 ;           Dn
      1 ;   2 ;     (1-Dn)*g
      1 ;   3 ; (1-Dn)*(1-g)
"

In this eqn object we specify a Two-High Threshold (2HT) model. Comments can be done with a # symbol at the beginning of a line.

The same model can be specified with the MDL syntax:

mdl = "
# CORE MODEL
## MDL SYNTAX

### targets
Do+(1-Do)*g
(1-Do)*(1-g)

### lure
(1-Dn)*g
Dn+(1-Dn)*(1-g)
"

All paths that lead to the same response category will be added together with a + symbol.

More information about these two syntaxes are written in Section 2.

The conversion to an rtmpt_model list can then be done by the to_rtmpt_model() function:

library(rtmpt)
# using the MDL syntax:
TwoHTM <- to_rtmpt_model(mdl_file = mdl)
# using the EQN syntax:
TwoHTM <- to_rtmpt_model(eqn_file = eqn)
TwoHTM

## 
## MODEL OVERVIEW
## 
## 
## MDL syntax for the MPT part:
##         MDL_lines
## 1   # 0 [ 0 , 1 ]
## 2     Do+(1-Do)*g
## 3    (1-Do)*(1-g)
## 4                
## 5   # 1 [ 2 , 3 ]
## 6        (1-Dn)*g
## 7 Dn+(1-Dn)*(1-g)
## 
## * NOTE 1: Each line in the MDL syntax represents one response category.
## ----------------------------
## 
## Process probability parameters (thetas):
##   Dn Do  g
## 1 NA NA NA
## 
## * NOTE 1: NA means the parameter will be estimated.
## * NOTE 2: A value larger than 0 and smaller than 1 means it will be held constant.
## * INFO: 0 of the process probabilities will be held constant.
## -------------------------------
## 
## Process completion time parameters (taus):
##       Dn Do  g
## minus NA NA NA
## plus  NA NA NA
## 
## * NOTE 1: "minus" refers to the negative outcome (1-P) and "plus" to the positive.
## * NOTE 2: NA means the parameter will be estimated. 0 means it will be suppressed.
## * INFO: 0 of the process completion times will be suppressed.
## -----------------------------------
## 
## Mapping of response categories and encoding plus motor execution times (deltas):
##   TREE CAT MAP
## 1    0   0   0
## 3    0   1   0
## 5    1   2   0
## 4    1   3   0
## 
## * NOTE 1: Unique representation of trees and categories.
## * NOTE 2: Each mapping number corresponds to a distinct encoding plus motor execution time.
## * INFO: 1 distinct delta(s) assumed, namely with mapping [ 0 ].
## -----------------------------------

1.2 Model restrictions

There are five functions with which an rtmpt_model object can be changed. Two functions can be used to hold a process probability parameter (theta) constant or to make some process probability parameters equal. Two functions can be used to suppress process completion times (taus), which means the process times are set to zero, or to make some process completion times equal. The last function handles how many different motor and execution times (deltas) are used in the model.

Modifying process probabilities

To hold a process probability constant you can use theta2const() like this:

theta2const(model = TwoHTM, names = "g", constants = 0.5)

## 
## MODEL OVERVIEW
## 
## 
## MDL syntax for the MPT part:
##         MDL_lines
## 1   # 0 [ 0 , 1 ]
## 2     Do+(1-Do)*g
## 3    (1-Do)*(1-g)
## 4                
## 5   # 1 [ 2 , 3 ]
## 6        (1-Dn)*g
## 7 Dn+(1-Dn)*(1-g)
## 
## * NOTE 1: Each line in the MDL syntax represents one response category.
## ----------------------------
## 
## Process probability parameters (thetas):
##   Dn Do   g
## 1 NA NA 0.5
## 
## * NOTE 1: NA means the parameter will be estimated.
## * NOTE 2: A value larger than 0 and smaller than 1 means it will be held constant.
## * INFO: 1 of the process probabilities will be held constant.
## -------------------------------
## 
## Process completion time parameters (taus):
##       Dn Do  g
## minus NA NA NA
## plus  NA NA NA
## 
## * NOTE 1: "minus" refers to the negative outcome (1-P) and "plus" to the positive.
## * NOTE 2: NA means the parameter will be estimated. 0 means it will be suppressed.
## * INFO: 0 of the process completion times will be suppressed.
## -----------------------------------
## 
## Mapping of response categories and encoding plus motor execution times (deltas):
##   TREE CAT MAP
## 1    0   0   0
## 3    0   1   0
## 5    1   2   0
## 4    1   3   0
## 
## * NOTE 1: Unique representation of trees and categories.
## * NOTE 2: Each mapping number corresponds to a distinct encoding plus motor execution time.
## * INFO: 1 distinct delta(s) assumed, namely with mapping [ 0 ].
## -----------------------------------

Here, "g" will be set to be equal to 0.5 and will not be estimated therefore.,You might also use this function with a vector of names (and a vector of constants). Note that the new model is not saved because we did not assign it to an R object.

To set two or more process probabilities equal you can use theta2theta() like this:

theta2theta(model = TwoHTM, names = c("Do", "Dn"), keep_consts = FALSE)

## 
## MODEL OVERVIEW
## 
## 
## MDL syntax for the MPT part:
##         MDL_lines
## 1   # 0 [ 0 , 1 ]
## 2     Do+(1-Do)*g
## 3    (1-Do)*(1-g)
## 4                
## 5   # 1 [ 2 , 3 ]
## 6        (1-Dn)*g
## 7 Dn+(1-Dn)*(1-g)
## 
## * NOTE 1: Each line in the MDL syntax represents one response category.
## ----------------------------
## 
## Process probability parameters (thetas):
##   Dn Do  g
## 1 NA Dn NA
## 
## * NOTE 1: NA means the parameter will be estimated.
## * NOTE 2: A value larger than 0 and smaller than 1 means it will be held constant.
## * INFO: 1 of the process probabilities will be held constant.
## -------------------------------
## 
## Process completion time parameters (taus):
##       Dn Do  g
## minus NA NA NA
## plus  NA NA NA
## 
## * NOTE 1: "minus" refers to the negative outcome (1-P) and "plus" to the positive.
## * NOTE 2: NA means the parameter will be estimated. 0 means it will be suppressed.
## * INFO: 0 of the process completion times will be suppressed.
## -----------------------------------
## 
## Mapping of response categories and encoding plus motor execution times (deltas):
##   TREE CAT MAP
## 1    0   0   0
## 3    0   1   0
## 5    1   2   0
## 4    1   3   0
## 
## * NOTE 1: Unique representation of trees and categories.
## * NOTE 2: Each mapping number corresponds to a distinct encoding plus motor execution time.
## * INFO: 1 distinct delta(s) assumed, namely with mapping [ 0 ].
## -----------------------------------

The process probabilities for all processes specified in names are set equal. The first of the process names in alphabetical order (here "dn") denotes the reference probability that will be estimated. The other(s) will not be estimated but will be set to be equal to the reference process probability. The keep_consts argument can be used, if you want to keep constants that you already specified and all other process probabilities should have the same constant but we suggest to use theta2const() for that.

Modifying process completion times

To suppress a process completion time (i.e., set it to zero) you can use tau2zero() like this:

tau2zero(model = TwoHTM, names = "g", outcomes = "minus", values = 0)

## 
## MODEL OVERVIEW
## 
## 
## MDL syntax for the MPT part:
##         MDL_lines
## 1   # 0 [ 0 , 1 ]
## 2     Do+(1-Do)*g
## 3    (1-Do)*(1-g)
## 4                
## 5   # 1 [ 2 , 3 ]
## 6        (1-Dn)*g
## 7 Dn+(1-Dn)*(1-g)
## 
## * NOTE 1: Each line in the MDL syntax represents one response category.
## ----------------------------
## 
## Process probability parameters (thetas):
##   Dn Do  g
## 1 NA NA NA
## 
## * NOTE 1: NA means the parameter will be estimated.
## * NOTE 2: A value larger than 0 and smaller than 1 means it will be held constant.
## * INFO: 0 of the process probabilities will be held constant.
## -------------------------------
## 
## Process completion time parameters (taus):
##       Dn Do  g
## minus NA NA  0
## plus  NA NA NA
## 
## * NOTE 1: "minus" refers to the negative outcome (1-P) and "plus" to the positive.
## * NOTE 2: NA means the parameter will be estimated. 0 means it will be suppressed.
## * INFO: 1 of the process completion times will be suppressed.
## -----------------------------------
## 
## Mapping of response categories and encoding plus motor execution times (deltas):
##   TREE CAT MAP
## 1    0   0   0
## 3    0   1   0
## 5    1   2   0
## 4    1   3   0
## 
## * NOTE 1: Unique representation of trees and categories.
## * NOTE 2: Each mapping number corresponds to a distinct encoding plus motor execution time.
## * INFO: 1 distinct delta(s) assumed, namely with mapping [ 0 ].
## -----------------------------------

Here, the process completion time of "g" with outcome "minus" (meaning the time for 1-g) is set to zero and will not be estimated therefore (which typically does not make sense to do). Also this function can be used with vectors.

To set two or more process completion times with the same outcome equal you can use theta2theta() like this:

tau2tau(model = TwoHTM, names = c("Do", "Dn"), keep_zeros = FALSE)

## 
## MODEL OVERVIEW
## 
## 
## MDL syntax for the MPT part:
##         MDL_lines
## 1   # 0 [ 0 , 1 ]
## 2     Do+(1-Do)*g
## 3    (1-Do)*(1-g)
## 4                
## 5   # 1 [ 2 , 3 ]
## 6        (1-Dn)*g
## 7 Dn+(1-Dn)*(1-g)
## 
## * NOTE 1: Each line in the MDL syntax represents one response category.
## ----------------------------
## 
## Process probability parameters (thetas):
##   Dn Do  g
## 1 NA NA NA
## 
## * NOTE 1: NA means the parameter will be estimated.
## * NOTE 2: A value larger than 0 and smaller than 1 means it will be held constant.
## * INFO: 0 of the process probabilities will be held constant.
## -------------------------------
## 
## Process completion time parameters (taus):
##       Dn Do  g
## minus NA Dn NA
## plus  NA Dn NA
## 
## * NOTE 1: "minus" refers to the negative outcome (1-P) and "plus" to the positive.
## * NOTE 2: NA means the parameter will be estimated. 0 means it will be suppressed.
## * INFO: 0 of the process completion times will be suppressed.
## -----------------------------------
## 
## Mapping of response categories and encoding plus motor execution times (deltas):
##   TREE CAT MAP
## 1    0   0   0
## 3    0   1   0
## 5    1   2   0
## 4    1   3   0
## 
## * NOTE 1: Unique representation of trees and categories.
## * NOTE 2: Each mapping number corresponds to a distinct encoding plus motor execution time.
## * INFO: 1 distinct delta(s) assumed, namely with mapping [ 0 ].
## -----------------------------------

The process completion times for all processes specified in names are set equal (i.e., all process completion times in minus-direction will be equal and all process completion times in plus-direction will be equal). The first of the process names in alphabetical order (here "dn") denotes the reference process times (in minus and plus direction) that will be estimated. The other(s) will not be estimated but will be set to be equal to the reference process times. The keep_zeros argument can be used, if you want to keep the zeros that you already specified and all other process times with the same outcome should be zero as well. Also here we suggest using the tau2zero() function.

Modifying motor and exectution times

You can choose which categories of the (RT-)MPT model should have the same encoding plus motor execution time. Unlike the processes the motor plus execution times have no names which we can use to set them equal. Therefore a mapping is used in the function delta2delta() where the response categories (categories) can be mapped to encoding plus motor execution times by using a mapping argument (mappings). Because the response categories are not always unique the tree (trees) has to be provided as well.

delta2delta(model = TwoHTM, trees = c(0,1), categories = c(1,3), mappings = c(1,1))

## 
## MODEL OVERVIEW
## 
## 
## MDL syntax for the MPT part:
##         MDL_lines
## 1   # 0 [ 0 , 1 ]
## 2     Do+(1-Do)*g
## 3    (1-Do)*(1-g)
## 4                
## 5   # 1 [ 2 , 3 ]
## 6        (1-Dn)*g
## 7 Dn+(1-Dn)*(1-g)
## 
## * NOTE 1: Each line in the MDL syntax represents one response category.
## ----------------------------
## 
## Process probability parameters (thetas):
##   Dn Do  g
## 1 NA NA NA
## 
## * NOTE 1: NA means the parameter will be estimated.
## * NOTE 2: A value larger than 0 and smaller than 1 means it will be held constant.
## * INFO: 0 of the process probabilities will be held constant.
## -------------------------------
## 
## Process completion time parameters (taus):
##       Dn Do  g
## minus NA NA NA
## plus  NA NA NA
## 
## * NOTE 1: "minus" refers to the negative outcome (1-P) and "plus" to the positive.
## * NOTE 2: NA means the parameter will be estimated. 0 means it will be suppressed.
## * INFO: 0 of the process completion times will be suppressed.
## -----------------------------------
## 
## Mapping of response categories and encoding plus motor execution times (deltas):
##   TREE CAT MAP
## 1    0   0   0
## 3    0   1   1
## 5    1   2   0
## 4    1   3   1
## 
## * NOTE 1: Unique representation of trees and categories.
## * NOTE 2: Each mapping number corresponds to a distinct encoding plus motor execution time.
## * INFO: 2 distinct delta(s) assumed, namely with mapping [ 0, 1 ].
## -----------------------------------

The numbers for the mappings have to start at zero and no whole number can be skipped. For example in the above code the mappings cannot be c(2,2) since the number 1 would be missing then in the mappings.

1.3 The right data format

The data should be provided with the following variables in this order: subj, group, tree, cat, and rt. All of these variables except rt should have values from zero to the number of unique values minus one. This will get clear with an example; if we have 60 subjects, two groups, two trees and four categories we would have the following values in the variables:

subj = {0,1,2,...,58,59},
group = {0,1},
tree = {0,1}, and
cat = {0,1,2,3}.

rt should be provided in milliseconds and as integers. If it has decimal places it will be rounded to a whole number.

Given the right order of the variables and the right values one can use a data.frame or a path to a text file containing the variable names in the first line. If not, the to_rtmpt_data() function can be used to reorder the variables and transform the values of those variables. Next to the new data.frame the transformation that is used will be saved in an rtmpt_data list. Note that at least the variable names should be correctly provided in order to make to_rtmpt_data() work. The to_rtmpt_data() function is also used automatically in the fit_rtmpt() function, but should be used at least once, when the model is specified the first time, to check whether the data is transformed according to the equation/model file.

Here is an example of a data set with unordered variables and wrong values (not starting from zero) and a transformation of this data set:

set.seed(2021)
raw_data <- data.frame(tree = rep(1:2, 8), rt = round(1000*(runif(16)+.3)), 
                       group = rep(1:2, each=8), subj = rep(1:4, each = 4), cat = rep(1:4, 4))
raw_data

##    tree   rt group subj cat
## 1     1  751     1    1   1
## 2     2 1084     1    1   2
## 3     1 1010     1    1   3
## 4     2  682     1    1   4
## 5     1  936     1    2   1
## 6     2 1001     1    2   2
## 7     1  940     1    2   3
## 8     2  567     1    2   4
## 9     1 1115     2    3   1
## 10    2 1283     2    3   2
## 11    1  327     2    3   3
## 12    2 1137     2    3   4
## 13    1  903     2    4   1
## 14    2  867     2    4   2
## 15    1 1120     2    4   3
## 16    2  552     2    4   4

data <- to_rtmpt_data(raw_data = raw_data, model = TwoHTM)
data

## 
## DATA TRANSFORMATION OVERVIEW
## 
## 
## Reordered variables:
## subj, group, tree, cat, rt
## * NOTE1: Additional variables are attached next to these five.
## * NOTE2: To see your data frame use <object name>$data.
## --------------------
## 
## Transformed variable(s):
## "subj"
##   old new
## 1   1   0
## 2   2   1
## 3   3   2
## 4   4   3
## 
## "group"
##   old new
## 1   1   0
## 2   2   1
## 
## "tree"
##   old new
## 1   1   0
## 2   2   1
## 
## "cat"
##   old new
## 1   1   0
## 2   3   1
## 3   2   2
## 4   4   3
## 
## * NOTE: "old" refers to the used labels and "new" to the ones that will be used.
## ------------------------

Note that it is never a good idea to specify the tree and category labels/numbers differently from the data file, or vice versa. It is bad practice to define a model with tree and category values starting from zero and using a data set with tree and category values starting from one; Make sure the labels or numbers match for these two match.

1.4 Model fitting

Finally, with the function fit_rtmpt() one can fit the model to the data. This code should not be run. There are way too few data points.

## do not run
rtmpt_out <- fit_rtmpt(model = TwoHTM, data = data)
## end not run

In this function call we used the default values for all the parameters that are not listed in the call. model and data do not have default values. The full call of the function would look like this:

## do not run
rtmpt_out <- fit_rtmpt(model = restr_2HTM, 
                       data = data, 
                       n.chains = 4, 
                       n.iter = 5000, 
                       n.burnin = 200,
                       n.thin = 1, 
                       Rhat_max = 1.05, 
                                 Irep = 1000, 
                       prior_params = NULL,
                       indices = FALSE,
                       save_log_lik = FALSE)
## end not run

n.chains is the number of chains with a maximum of 16, n.iter is the number of samples to draw from the posterior distribution, n.burnin is analogue to the burn-in phase for R package rjags, Rhat_max is a threshold for the potential scale reduction factor and forces the sampling to start only when the maximal potential scale reduction factor of the parameters is lower or equal to this value, n.thin is the thinning parameter, Irep is the number of samples until the next summary is printed, prior_params is a list of some prior parameter specifications, indices is a logical parameter with which one can allow to compute the WAIC and LOO, and save_log_lik is a logical parameter that allows one to save the log-likelihood for each posterior sample and data point.

1.5 Checking model convergence and getting a summary

Convergence of the model can be checked by using the fitted object (here rtmpt_out). Just get the diagnostics and then R-hat. In order to get a traceplot one can use the traceplot() function from coda. You can also use the summary() function to get a summary of the estimates.

rtmpt_out$diags$R_hat
coda::traceplot(rtmpt_out$samples[, 1:9])
summary(rtmpt_out)

2 Equation and model files

In previous versions it was possible to set some restrictions directly in the equation/model file, but due to the number of restrictions that are possible we decided to use now functions instead (see Subsection 1.2).

2.1 Equation files

As we have seen above one can specify an equation file (eqn_file) like this:

eqn = "
# CORE MODEL
## tree ; cat ;         path
      0 ;   0 ;           Do
      0 ;   0 ;     (1-Do)*g
      0 ;   1 ; (1-Do)*(1-g)

      1 ;   3 ;           Dn
      1 ;   2 ;     (1-Dn)*g
      1 ;   3 ; (1-Dn)*(1-g)
"

The trees, categories, and paths need to be separated by a semi-colon or a comma, but should not be mixed. As mentioned above the eqn_file can be provided by a text file through a path to the file or written directly in R/Rstudio like above.

2.2 Model files

Another way of specifying the same model is by providing a model file (mdl_file). This is also not yet the model itself as needed for the fit_rtmpt() call, but much closer. It is based on the syntax developed by Singmann and Kellen (2013).

mdl = "
# CORE MODEL
## targets:
Do+(1-Do)*g
(1-Do)*(1-g)

## lures
(1-Dn)*g
Dn+(1-Dn)*(1-g)
"

Between two trees (here targets and lures) there must be an empty line. All paths which lead to the same response category are added together on one line by a + symbol.

Internally an eqn_file will be converted to an mdl_file. From that a text file will be written locally onto your computer, used by the fit_rtmpt() call, and removed afterwards.

3 Data transformation

3.1 Labels in data file

When labels are used in the data set one has to be cautious when specifying the equation file. The labels of the trees and categories in the equation file must match those of the data. For example, if the tree labels “target” and “lure” are used in the data set, one has to use those also in the equation file (eqn_file). If you specify a model file (mdl_file) instead of the equation file the labels in the data set need to be changed to numerics. Otherwise no mapping between those can be done.

3.2 Numbers in data file

When numbers are used in the data set for the trees and categories it is best to also check that they match with the ones used in the equation / model file, but the program checks for the order. So for example, if you are using the numbers {1, 2, 3, 4} for your four trees in the data set and {0, 1, 2, 3} in the equation file (which is the standard for model files anyway) they will be mapped through the order. The new data, that can be used, will have a zero instead of a one, a one instead of a two, and so on.

One could use labels in the equation file even though you are using numbers in the data file, but it is strongly advised against it. It will be assumed, that the order in which you specify the equations matches the order of the numbers in the data file starting from the lowest to the highest. So if you are using again {1, 2, 3, 4} for the four trees, then the first tree you need to define in the equation files (no matter what label you use) must correspond to tree 1 in the data and so on.

4 Data simulation and simulation-based calibration

Data simulation

With the function sim_rtmpt_data() it is possible to generate data from an RT-MPT model. Required is a model object generated with the function to_rtmpt_model(), a random seed number, the number of subjects, the number of trials per tree in the model, and a named list of parameters. Here it is also possible to set the mean of mean_of_mu_alpha; A value of zero refers to the probability of 0.5. If a given probability is desired, say 0.4, then one might specify params <- list(mean_of_mu_alpha = qnorm(.4)). If a mean process time of 200 ms is required, one might specify params <- list(mean_of_exp_mu_beta = 1000/200). The mean parameter of the motor times is easier to transform, since they are already on the second scale; Just write 500/1000 if your mean motor time should be 500 ms. The code below shows a more or less good balance between too large variances (leading to too many extreme values like probabilities of zero and one and RTs of zero) and too small variances (unrealistic data).

Important: If no variances are specified in the params argument, then the corresponding mean values are fixed, else they will be randomly drawn.

# randomly drawn group-level mean values
mdl_2HTM <- "
# targets
do+(1-do)*g     ; 0
(1-do)*(1-g)    ; 1

# lures
(1-dn)*g        ; 0
dn+(1-dn)*(1-g) ; 1

# do: detect old; dn: detect new; g: guess
"

model <- to_rtmpt_model(mdl_file = mdl_2HTM)

# random group-level parameters
params <- list(mean_of_mu_alpha = 0, 
               var_of_mu_alpha = 1,         # delete this line to fix mean_of_mu_alpha to 0
               mean_of_exp_mu_beta = 10, 
               var_of_exp_mu_beta = 10,     # delete this line to fix mean_of_exp_mu_beta to 10
               mean_of_mu_gamma = 0.5, 
               var_of_mu_gamma = 0.0025,    # delete this line to fix mean_of_mu_gamma to 0.5
               mean_of_omega_sqr = 0.005, 
               var_of_omega_sqr = 0.000025, # delete this line to fix mean_of_omega_sqr to 0.005
               df_of_sigma_sqr = 10, 
               sf_of_scale_matrix_SIGMA = 0.1, 
               sf_of_scale_matrix_GAMMA = 0.01, 
               prec_epsilon = 10,
               add_df_to_invWish = 5)

sim_dat <- sim_rtmpt_data(model, seed = 123, n.subj = 40, n.trials = 30, params = params)
head(sim_dat$data_frame)

##   subj group tree cat  rt
## 1    0     0    0   0 560
## 2    0     0    0   0 456
## 3    0     0    0   0 538
## 4    0     0    0   0 427
## 5    0     0    0   1 420
## 6    0     0    0   0 574

4.1 Simulation-based calibration

With the function fit_rtmpt_SBC() it is possible to generate data and fit the model to that data using the same priors / ground truths. The main output of this function will be the rank statistic for each parameter in the model, which is needed for the simulation-based calibration (SBC) method by Talts, Betancourt, Simpson, Vehtari, and Gelman (2018). For example, if this function is repeated, say 2000 times, with different seeds one might add all the rank statistics to a matrix (2000 rows and number of parameters as columns) and then calculate the pearsons’ chi-squared statistic. This allows one to test for how many of the parameters the rank statistics are not uniformly distributed. A value of about 0.05 is typical for a test with an alpha level of 0.05.

Here is an example of one replication of the SBC procedure:

mdl_2HTM <- "
# targets
d+(1-d)*g     ; 0
(1-d)*(1-g)    ; 1

# lures
(1-d)*g        ; 0
d+(1-d)*(1-g) ; 1

# d: detect; g: guess
"

model <- to_rtmpt_model(mdl_file = mdl_2HTM)

params <- list(mean_of_exp_mu_beta = 10, 
               var_of_exp_mu_beta = 10, 
               mean_of_mu_gamma = 0.5, 
               var_of_mu_gamma = 0.0025, 
               mean_of_omega_sqr = 0.005, 
               var_of_omega_sqr = 0.000025,
               df_of_sigma_sqr = 10, 
               sf_of_scale_matrix_SIGMA = 0.1, 
               sf_of_scale_matrix_GAMMA = 0.01, 
               prec_epsilon = 10,
               add_df_to_invWish = 5)

SBC_out <- fit_rtmpt_SBC(model, seed = 123, prior_params = params)
SBC_out$ranks

With the following code it would essentially be possible to calculate the rank statistics of each parameter and then test it with the pearsons’ chi-squared test, but much is not considered here (e.g. the convergence and the effective sample size).

# For 2000 replications
## This takes too long to run and in addition, Rhat should always be 
## checked as well as the effective sample size.
R = 2000
rank_mat <- data.frame()

for (r in 1:R) {
  SBC_out <- fit_rtmpt_SBC(model, seed = r*123, prior_params = params, n.eff_samples = 99)
  rank_mat <- rbind(rank_mat, SBC_out$ranks)
}

## pearsons' chi-square for testing uniformity
x <- apply(rank_mat[1:R,], 2, table)
expect <- R/100 # 100 = number of bins/cells (0:99)
pearson <- apply(X = x, MARGIN = 2, FUN = function(x) {sum((x-expect)^2/expect)})
z95 <- qchisq(0.95, 99) # 99 = degrees of freedom
sum(pearson>z95) / length(pearson)

With the following code a rank matrix is generated from a uniform distribution and then tested for uniformity:

R <- 2000
rand_rankmat <- matrix(data = sample(0:99, R*393, replace=TRUE), nrow = R, ncol = 393)

## pearsons' chi-square for testing uniformity
x <- apply(rand_rankmat[1:R,], 2, table)
expect <- R/100 # 100 = number of bins/cells (0:99)
pearson <- apply(X = x, MARGIN = 2, FUN = function(x) {sum((x-expect)^2/expect)})
z95 <- qchisq(0.95, 99) # 99 = degrees of freedom
sum(pearson>z95) / length(pearson)

## [1] 0.05343511

References

Hartmann, R., Johannsen, L., & Klauer, K. C. (2020). rtmpt: An R package for fitting response-time extended multinomial processing tree models. Behavior Research Methods, 52(3), 1313-1338. doi:10.3758/s13428-019-01318-x
Hartmann, R., & Klauer, K. C. (2020). Extending RT-MPTs to enable equal process times. Journal of Mathematical Psychology, 96, 102340. doi:10.1016/j.jmp.2020.102340
Heck, D. W., Arnold, N. R., & Arnold, D. (2018). TreeBUGS: An R package for hierarchical multinomial-processing-tree modeling. Behavior Research Methods, 50(1), 264-284. doi:10.3758/s13428-017-0869-7
Hu, X. (1999). Multinomial processing tree models: An implementation. Behavior Research Methods, Instruments, & Computers, 31(4), 689-695. doi:10.3758/BF03207714
Klauer, K. C., & Kellen, D. (2018). RT-MPTs: Process models for response-time distributions based on multinomial processing trees with applications to recognition memory. Journal of Mathematical Psychology, 82, 111-130. doi:10.1016/j.jmp.2017.12.003
Singmann, H., & Kellen, D. (2013). MPTinR: Analysis of multinomial processing tree models in R. Behavior Research Methods, 45(2), 560-575. doi:10.3758/s13428-012-0259-0
Talts, S., Betancourt, M., Simpson, D., Vehtari, A., & Gelman, A. (2018). Validating Bayesian inference algorithms with simulation-based calibration. arXiv preprint arXiv:1804.06788.

rtmpt: Guideline

Raphael Hartmann

2023-11-30