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The real use case

In neuroimaging and multivariate biomedical research, it is common to run the same model across many features simultaneously — brain regions, EEG channels, metabolites, or questionnaire subscales. ptestR is designed for exactly this pattern via the nest() + map() + unnest() idiom from the tidyverse.


Simulated dataset

We simulate a dataset with 15 subjects, each measured at 3 time points, across 2 groups and 4 features. This gives 45 observations per feature — enough for lmer to fit random intercepts per subject comfortably.

set.seed(42)

n_subj    <- 15
n_time    <- 3
features  <- c("feature_A", "feature_B", "feature_C", "feature_D")

# Subject-level covariates
subjects <- data.frame(
  subject = seq_len(n_subj),
  group   = rep(c("control", "case"), length.out = n_subj),
  age     = round(rnorm(n_subj, mean = 35, sd = 8), 1)
)

# Expand to subject × time × feature, then add values
df <- expand.grid(
  subject = seq_len(n_subj),
  time    = seq_len(n_time),
  feature = features,
  stringsAsFactors = FALSE
) |>
  left_join(subjects, by = "subject") |>
  mutate(
    # Random intercept per subject (shared across features)
    rand_int = rep(rnorm(n_subj, sd = 1), times = n_time * length(features)),
    value = case_when(
      feature == "feature_A" ~  0.6 * (group == "case") + 0.2 * time + rand_int + rnorm(n()),
      feature == "feature_B" ~  0.1 * (group == "case") + 0.1 * time + rand_int + rnorm(n()),
      feature == "feature_C" ~ -0.5 * (group == "case") + 0.3 * time + rand_int + rnorm(n()),
      feature == "feature_D" ~  0.0 * (group == "case") + 0.0 * time + rand_int + rnorm(n())
    )
  ) |>
  select(subject, time, group, age, feature, value)

glimpse(df)
#> Rows: 180
#> Columns: 6
#> $ subject <int> 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 1, 2, 3, 4,
#> $ time    <int> 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2,
#> $ group   <chr> "control", "case", "control", "case", "control", "case", "cont…
#> $ age     <dbl> 46.0, 30.5, 37.9, 40.1, 38.2, 34.2, 47.1, 34.2, 51.1, 34.5, 45…
#> $ feature <chr> "feature_A", "feature_A", "feature_A", "feature_A", "feature_A…
#> $ value   <dbl> 1.29140052, 1.22058442, -1.42135190, -2.24939330, 2.02506847, 

Each nested sub-dataset (one per feature) will have n_subj × n_time = 45 rows with 15 distinct subjects — well within lmer’s requirements.


Running across features with grouped_perm_glmm()

The pattern is: group_by() the feature → nest()map() the permutation function → unnest() the results.

results <- df |>
  group_by(feature) |>
  nest() |>
  mutate(
    perm = map(data, \(d) grouped_perm_glmm(
      tbl         = d,
      formla      = value ~ group + age + time + (1 | subject),
      var_to_perm = "value",
      permNum     = 499,
      seed        = 42
    ))
  ) |>
  unnest(perm) |>
  select(-data)
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results
#> # A tibble: 16 × 6
#> # Groups:   feature [4]
#>    feature   term         effect estimate statistic  p.perm
#>    <chr>     <chr>        <chr>     <dbl>     <dbl>   <dbl>
#>  1 feature_A (Intercept)  fixed   -0.899     -0.484 0.595  
#>  2 feature_A age          fixed    0.0190     0.403 0.685  
#>  3 feature_A groupcontrol fixed   -0.897     -1.20  0.220  
#>  4 feature_A time         fixed    0.484      2.82  0.00401
#>  5 feature_B (Intercept)  fixed   -1.71      -0.784 0.435  
#>  6 feature_B age          fixed    0.0341     0.617 0.517  
#>  7 feature_B groupcontrol fixed   -0.324     -0.369 0.705  
#>  8 feature_B time         fixed    0.221      1.05  0.321  
#>  9 feature_C (Intercept)  fixed   -2.62      -1.04  0.259  
#> 10 feature_C age          fixed    0.0231     0.359 0.683  
#> 11 feature_C groupcontrol fixed    0.466      0.455 0.649  
#> 12 feature_C time         fixed    0.769      4.32  0      
#> 13 feature_D (Intercept)  fixed   -1.96      -1.18  0.265  
#> 14 feature_D age          fixed    0.0481     1.14  0.242  
#> 15 feature_D groupcontrol fixed   -0.505     -0.754 0.469  
#> 16 feature_D time         fixed   -0.0504    -0.342 0.758

Filtering to a term of interest

Filter to the group effect across all features:

group_results <- results |>
  filter(term == "groupcontrol") |>
  arrange(p.perm)

group_results |>
  select(feature, estimate, statistic, p.perm)
#> # A tibble: 4 × 4
#> # Groups:   feature [4]
#>   feature   estimate statistic p.perm
#>   <chr>        <dbl>     <dbl>  <dbl>
#> 1 feature_A   -0.897    -1.20   0.220
#> 2 feature_D   -0.505    -0.754  0.469
#> 3 feature_C    0.466     0.455  0.649
#> 4 feature_B   -0.324    -0.369  0.705

Feature A and C should show the strongest effects (we simulated them that way), while feature D should be near noise.


FDR correction

With multiple features, apply FDR correction across all models for each term:

group_results <- group_results |>
  mutate(p.fdr = p.adjust(p.perm, method = "fdr"))

group_results |>
  select(feature, estimate, statistic, p.perm, p.fdr)
#> # A tibble: 4 × 5
#> # Groups:   feature [4]
#>   feature   estimate statistic p.perm p.fdr
#>   <chr>        <dbl>     <dbl>  <dbl> <dbl>
#> 1 feature_A   -0.897    -1.20   0.220 0.220
#> 2 feature_D   -0.505    -0.754  0.469 0.469
#> 3 feature_C    0.466     0.455  0.649 0.649
#> 4 feature_B   -0.324    -0.369  0.705 0.705

Handling p.perm = 0

When no permuted statistic is as extreme as the observed one, p.perm = 0. This is not a true zero — it means the effect is stronger than all permNum permuted null statistics. Report as p < 1/permNum:

group_results |>
  mutate(
    p.label = ifelse(p.perm == 0,
                     paste0("p < ", round(1 / 499, 4)),
                     paste0("p = ", round(p.perm, 3)))
  ) |>
  select(feature, p.label, p.fdr)
#> # A tibble: 4 × 3
#> # Groups:   feature [4]
#>   feature   p.label   p.fdr
#>   <chr>     <chr>     <dbl>
#> 1 feature_A p = 0.22  0.220
#> 2 feature_D p = 0.469 0.469
#> 3 feature_C p = 0.649 0.649
#> 4 feature_B p = 0.705 0.705

With permNum = 9999 (recommended for publication), the bound becomes p < 0.0001.


Plotting the results

A simple dot plot showing effect size and significance:

if (requireNamespace("ggplot2", quietly = TRUE)) {
  library(ggplot2)

  group_results |>
    mutate(sig = p.fdr < 0.05) |>
    ggplot(aes(x = estimate, y = feature, colour = sig)) +
    geom_vline(xintercept = 0, linetype = "dashed", colour = "#DCC3AA") +
    geom_point(size = 4) +
    scale_colour_manual(
      values = c("TRUE" = "#810B38", "FALSE" = "#DCC3AA"),
      labels = c("TRUE" = "FDR < 5%", "FALSE" = "ns"),
      name   = NULL
    ) +
    labs(
      title = "Group effect by feature",
      x     = "Coefficient estimate",
      y     = NULL
    ) +
    theme_minimal(base_size = 12)
}


Using grouped_perm_glm() instead

If your data have no repeated measures, swap to grouped_perm_glm() and drop the random-effects term:

# Summarise to one observation per subject × feature (e.g. mean across time)
df_subj <- df |>
  group_by(subject, group, age, feature) |>
  summarise(value = mean(value), .groups = "drop")

glm_results <- df_subj |>
  group_by(feature) |>
  nest() |>
  mutate(
    perm = map(data, \(d) grouped_perm_glm(
      tbl         = d,
      formla      = value ~ group + age,
      var_to_perm = "value",
      permNum     = 499,
      seed        = 42
    ))
  ) |>
  unnest(perm) |>
  select(-data)

glm_results |>
  filter(term == "groupcontrol") |>
  select(feature, estimate, statistic, p.perm) |>
  arrange(p.perm)
#> # A tibble: 4 × 4
#> # Groups:   feature [4]
#>   feature   estimate statistic p.perm
#>   <chr>        <dbl>     <dbl>  <dbl>
#> 1 feature_A   -0.897    -1.20   0.228
#> 2 feature_D   -0.505    -0.754  0.469
#> 3 feature_C    0.466     0.455  0.659
#> 4 feature_B   -0.324    -0.369  0.683

Tips for large-scale analyses

Use more permutations for publication. 499 is used here for speed; use 9 999 or more for final results.

Parallelise with furrr. Each map() call is independent — swap purrr::map() for furrr::future_map() for free parallelisation:

library(furrr)
plan(multisession, workers = 4)

results <- df |>
  group_by(feature) |>
  nest() |>
  mutate(
    perm = future_map(data, \(d) grouped_perm_glmm(
      d,
      formla      = value ~ group + age + time + (1 | subject),
      var_to_perm = "value",
      permNum     = 9999,
      seed        = 42
    ), .options = furrr_options(seed = TRUE))
  ) |>
  unnest(perm) |>
  select(-data)