Typical Use of `eatATA`: a Pilot Study Example"
In eatATA: Create Constraints for Small Test Assembly Problems

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

eatATA efficiently translates test design requirements for Automated Test Assembly (ATA) into constraints for a Mixed Integer Linear Programming Model (MILP). A number of efficient and user-friendly functions are available that translate conceptual test assembly constraints to constraint objects for MILP solvers, like the GLPK solver. In the remainder of this vignette I will illustrate the typical use of eatATA using a case-based example. A general overview over eatATA can be found in the vignette Overview of eatATA Functionality.

Setup

The eatATA package can be installed from CRAN.

install.packages("eatATA")

First, eatATA is loaded into the R session.

# loading eatATA
library(eatATA)

Item pool

No ATA without an item pool. In this example we use a fictional example item pool of 80 items. The item pool information is stored as an excel file that is included in the package. To import the item pool information into R we recommend using the package readxl. This package imports the data as a tibble, but in the code below, the item pool is immediately transformed into a data.frame.

Note that R requires a rectangular data set. Yet, often excel files store additional information in rows above or below the "rectangular" item pool information. The skip argument in the read_excel() function can be used to skip unnecessary rows in the excel file. (Note that the item pool can also be directly accessed in the package via items; see ?items for more information.)

items_path <- system.file("extdata", "items.xlsx", package = "eatATA")

items <- as.data.frame(readxl::read_excel(path = items_path), stringsAsFactors = FALSE)

Inspection of the item pool indicates that the items have different properties: item format (MC, CMC, short_answer, or open), difficulty (diff_1 - diff_5), average response times in minutes (time). In addition, similar items can not be in the same booklet or test form. This information is stored in the column exclusions, which indicates which items are too similar and should not be in the same booklet with the item in that row..

head(items)

Prepare item information

Before defining the constraints, item pool data has to be in the correct format. For instance, some dummy variables (indicator variables) in the item pool use both NA and 0 to indicate "the category does not apply". Therefore, the dummy variables should be transformed so that there are only two values (1 = "the category applies", and 0 = "the category does not apply").

Often a set of dummy variables can be summarized into a single factor variable. This is automatically done by the function dummiesToFactor(). However, the function can only be used when the categories are mutually exclusive. For instance, in the example item pool, items can contain sub-items with different format or difficulties. As a result, some items contain two sub-items with different formats. Therefore, in this example, the dummiesToFactor() function throws an error and cannot be used.

# clean data set (categorical dummy variables must contain only 0 and 1)
items <- dummiesToFactor(items, dummies = c("MC", "CMC", "short_answer", "open"), facVar = "itemFormat")
items <- dummiesToFactor(items, dummies = paste0("diff_", 1:5), facVar = "itemDiff")
items[c(24, 33, 37, 47, 48, 54, 76), ]

In addition, the column short_answer can have r max(items$short_answer) as a value, and is consequently not a dummy variable. Therefore, we will (a) treat short_answer as a numerical value, (b) collapse MC and open into a new factor MC_open_none, (these dummies are mutually exclusive), and (c) turn CMC and the difficulty indicators into factors. (See ?autoItemValuesMinMax and ?computeTargetValues for further information on the different treatment of factors and numerical variables.)

# make new factor with three levels: "MC", "open" and "else"
items <- dummiesToFactor(items, dummies = c("MC", "open"), facVar = "MC_open_none")
# clean data set (NA should be 0)
for(ty in c(paste0("diff_", 1:5), "CMC", "short_answer")){
  items[, ty] <- ifelse(is.na(items[, ty]), yes = 0, no = items[, ty])
}
# make factors of CMC dummi
items$f_CMC <- factor(items$CMC, labels = paste("CMC", c("no", "yes"), sep = "_"))

# example item format
table(items$short_answer)

`ATA` goal

In this example, the goal is to assemble 14 booklets out of the 80 items item pool. All items should be assigned to one (and only one booklet), so that there is no item overlap and the item pool is completely depleted.

To be more precise, the required constraints are:

no item overlap between test blocks
complete item pool depletion
equal distribution of item formats across test blocks
equal difficulty distribution across test blocks
some items can not be together in the same booklet (item exclusions)
as similar as possible response times across booklets

Note that the booklets will later be manually assembled to test forms.

For ease of use, we set up two variables that we will use frequently: the number of test forms or booklets to be created (nForms) and the number of items in the item pool (nItems).

# set up fixed variables
nItems <- nrow(items)  # number of items
nForms <- 14           # number of blocks

eatATA offers a variety of functions that automatically compute the constraints mentioned above and which are illustrated in the following.

Objective Function

First, we are setting up an optimization constraint. This constraint is not a clear yes or no constraint, and it does not have to be attained perfectly. Instead, the solver will minimize the distance of the actual booklet value for all booklets towards a target value. In our example, we specify 10 minutes as the target response time time for all booklets.

# optimize average time
av_time <- minimaxObjective(nForms, itemValues = items$time, targetValue = 10,
                             itemIDs = items$item)

Set up constraints

The first two constraints (no item overlap and item pool depletion) can be implemented by a single function: itemUsageConstraint(). To achieve this, the operator argument should be set to "=", meaning that every item should be used exactly once in the booklet assembly.

itemOverlap <- itemUsageConstraint(nForms, targetValue = 1, 
                                   operator = "=", itemIDs = items$item)

Constraints with respect to categorical variables or factors (like MC_open_none) or numerical variables (like short_answer), can be easily implemented using the autoItemValuesMinMax() function. The result of this function depends on whether a factor or a numerical variable is used. That is, autoItemValuesMinMax() automatically determines the minimum and maximum frequency of each category of a factor. But for numerical variables, it automatically determines the target value.

The allowedDeviation argument specifies the allowed range between booklets regarding the category or the numerical value. If the argument is omitted, it defaults to "no deviation is allowed" for numerical values, and to the minimal possible deviation for categorical variables or factors. Hence, for numeric values, we specify allowedDeviation = 1. The function prints the calculated target value or the resulting allowed value range on booklet level.

# item formats
mc_openItems <- autoItemValuesMinMaxConstraint(nForms = nForms, itemValues = items$MC_open_none, 
                                     itemIDs = items$item)
cmcItems <- autoItemValuesMinMaxConstraint(nForms = nForms, itemValues = items$f_CMC, itemIDs = items$item)
saItems <- autoItemValuesMinMaxConstraint(nForms = nForms, itemValues = items$short_answer, 
                                allowedDeviation = 1, itemIDs = items$item)

# difficulty categories
Items1 <- autoItemValuesMinMaxConstraint(nForms = nForms, itemValues = items$diff_1, 
                               allowedDeviation = 1, itemIDs = items$item)
Items2 <- autoItemValuesMinMaxConstraint(nForms = nForms, itemValues = items$diff_2, 
                               allowedDeviation = 1, itemIDs = items$item)
Items3 <- autoItemValuesMinMaxConstraint(nForms = nForms, itemValues = items$diff_3, 
                               allowedDeviation = 1, itemIDs = items$item)
Items4 <- autoItemValuesMinMaxConstraint(nForms = nForms, itemValues = items$diff_4, 
                               allowedDeviation = 1, itemIDs = items$item)
Items5 <- autoItemValuesMinMaxConstraint(nForms = nForms, itemValues = items$diff_5, 
                               allowedDeviation = 1, itemIDs = items$item)

To implement item exclusion constraints, two function can be used: itemExclusionTuples() and itemExclusionConstraint(). When item exclusions are supplied as a single character string for each item, with item identifiers separated by ", ", they should be transformed first.

# item exclusions variable
items$exclusions[1:5]

This transformation can be done using the itemExclusionTuples() function, which creates so called tuples: pairs of exclusive items. These tuples can be used directly with the itemExclusionConstraint() function.

# item exclusions
exclusionTuples <- itemTuples(items, idCol = "item", 
                                       infoCol = "exclusions", sepPattern = ", ")
excl_constraints <- itemExclusionConstraint(nForms = 14, itemTuples = exclusionTuples, 
                                            itemIDs = items$item)

Another helpful function is the itemsPerFormConstraint() function, which constrains the number of items per booklet. However, since this is not required in this example, we will not use these constraints in the final ATA constraints.

# number of items per test form
min_Nitems <- floor(nItems / nForms) - 3
noItems <- itemsPerFormConstraint(nForms = nForms, operator = ">=", 
                                  targetValue = min_Nitems, itemIDs = items$item)

Run solver

Before calling the optimization algorithm the specified constraints are collected in a list.

# Prepare constraints
constr_list <- list(itemOverlap, mc_openItems, cmcItems, saItems, 
                      Items1, Items2, Items3, Items4, Items5, 
                      excl_constraints,
                      av_time)

Now we can call the useSolver() function, which restructures the constraints internally and solves the optimization problem. Using the solver argument we specify GLPK as the solver (other alternatives are lpSolve, Symphony and Gurobi). Using the timeLimit argument we set the time limit to 10. This means we limit the solver to stop searching for an optimal solution after 10 seconds. Note that the computation times might depend on the solver you have selected.

# Local optimization chunk; prevents occasional errors on servers if no feasible solution is found
output_local <- capture.output(solver_raw_local <- useSolver(constr_list, nForms = nForms, nItems = nItems, 
                        itemIDs = items$item, solver = "GLPK", timeLimit = 30))
saveRDS(solver_raw_local, "use_case_solver_out.RDS")
saveRDS(output_local, "use_case_output.RDS")

# Optimization
solver_raw <- useSolver(constr_list, nForms = nForms, nItems = nItems, 
                        itemIDs = items$item, solver = "GLPK", timeLimit = 10)

solver_raw <- readRDS("use_case_solver_out.RDS")
output_local <- readRDS("use_case_output.RDS")
output_local
message("The solution is feasible, but may not be optimal")

The function provides output that indicates whether an optimal solution has been found. In our case, a viable solution has been found but the function reached the time limit before finding the optimal solution.

If there is no feasible solution, one option is to relax some of the constraints. Further, for first diagnostic purposes you can omit some constraints completely, to see which constraints are especially challenging. If you have a better grasp of the possibilities of the item pool, you can add these constraints back, but for example with larger allowedDeviations.

Inspect Solution

The solution provided by eatATA can be inspected using the inspectSolution() function. It allows us to inspect the assembled item blocks at a first glance, including some column sums.

out_list <- inspectSolution(solver_raw, items = items, idCol = "item", colSums = TRUE,
                            colNames = c("time", "subitems", 
                                         "MC", "CMC", "short_answer", "open",
                                         paste0("diff_", 1:5)))

# first two booklets
out_list[1:2]

In our case we want to assemble the created booklets into test forms. Therefore, we are interested in booklet exclusions that can result from item exclusions. The analyzeBlockExclusion() function can be used to obtain tuples with booklet exclusions.

analyzeBlockExclusion(solverOut = solver_raw, item = items, idCol = "item", 
                      exclusionTuples = exclusionTuples)

Save as Excel

To save the item distribution on blocks or test forms, we can use the appendSolution() function. The function simply merges the new variables containing the solution to the test assembly problem to the original item pool.

out_df <- appendSolution(solver_raw, items = items, idCol = "item")

Finally, when the solution should be exported as an excel file (.xlsx), this can, for example, be achieved via the eatAnalysis package, which has to be installed from Github.

devtools::install_github("beckerbenj/eatAnalysis")

eatAnalysis::write_xlsx(out_df, filePath = "example_excel.xlsx",
                        row.names = FALSE)