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CERAD Constructional Praxis Scoring

Source: vignettes/cerad-scoring.Rmd
cerad-scoring.Rmd

What cat.cog does

cat.cog automates CERAD Constructional Praxis scoring using vision-capable LLMs. CERAD is a widely-used cognitive assessment in dementia research; one of its subtests asks subjects to copy four geometric shapes (circle, diamond, two overlapping rectangles, cube). Standard scoring assigns 0–11 points based on visible drawing features following published rubrics.

cerad_drawn_score() sends each image to a vision model, classifies the relevant drawing features (closure, symmetry, intersection, depth cues, etc.), then applies the CERAD scoring rules to return a numeric score per drawing.

The function is a thin wrapper over cat.stack::classify() with image input, plus the CERAD-specific scoring logic baked in.

Install 

install.packages(
  "cat.cog",
  repos = c("https://chrissoria.r-universe.dev",
            "https://cloud.r-project.org")
)
library(cat.cog)

Score a directory of drawings 

scores <- cerad_drawn_score(
  shape       = "circle",
  image_input = "./circle_drawings/",       # directory of PNG/JPG files
  api_key     = Sys.getenv("OPENAI_API_KEY"),
  user_model  = "gpt-4o"                    # vision-capable
)

head(scores[, c("image_file", "score")])

The returned data.frame has one row per image with the integer score, the raw classification of each scoring feature, and the image filename for joining back to participant records.

Score a vector of individual files 

drawing_paths <- c(
  "./participant_001_circle.png",
  "./participant_002_circle.png",
  "./participant_003_circle.png"
)

scores <- cerad_drawn_score(
  shape       = "circle",
  image_input = drawing_paths,
  api_key     = Sys.getenv("OPENAI_API_KEY"),
  user_model  = "gpt-4o"
)

The four CERAD shapes 

shapes <- c("circle", "diamond", "rectangles", "cube")
max_scores <- c(circle = 2, diamond = 3, rectangles = 2, cube = 4)
# Total possible: 11

for (s in shapes) {
  scores <- cerad_drawn_score(
    shape       = s,
    image_input = file.path("./drawings", s),
    api_key     = Sys.getenv("OPENAI_API_KEY"),
    user_model  = "gpt-4o"
  )
  saveRDS(scores, paste0("./scores_", s, ".rds"))
}

Image preparation tips

  1. Scan or photograph cleanly. Crop tightly to the drawing, correct rotation, and ensure good contrast (dark drawing on light background). Background noise (rulers, stamps, the original reference shape on the same page) can mislead the model.
  2. Consistent format. PNG with white background works best. Convert JPEGs from cameras if possible.
  3. One shape per image. Do not include the reference shape next to the subject’s drawing — the model may not distinguish them.
  4. Resolution. ~500–1000 px on the long edge is plenty; larger files just cost more tokens without improving accuracy.

Use a strong vision model

Smaller models often miss subtle scoring features (e.g., whether rectangles actually intersect, whether the cube has visible depth lines). For research use, gpt-4o or claude-3-5-sonnet is recommended over the mini/haiku tier.

Validation

Always benchmark against expert human scoring on a subsample before trusting LLM scores at scale. Compute weighted Cohen’s κ between the LLM and a clinician — values around 0.7+ are generally acceptable for secondary analysis; pre-registered primary outcomes warrant higher agreement.

Where to learn more