feat: cooked up something fierce

src/analysis/classical.rs

@@ -134,6 +134,14 @@ pub struct ItemAnalysis {
     pub number: u32,
     /// The item's global id, when known.
     pub item_ref: Option<String>,
+    /// The option set administered, when the rows agree on one.
+    ///
+    /// Within one administration an item has one variant, because a placement's
+    /// distractors are drawn once and shared by every form — only the printed
+    /// order differs. `None` means the rows disagreed, which happens when a
+    /// course opts into drawing distractors per form; the statistics below then
+    /// describe a mixture and cannot be pooled by option set.
+    pub variant: Option<String>,
     /// How many students the item was administered to.
     pub n: usize,
     /// How many gave a non-blank response.
@@ -553,6 +561,7 @@ pub fn analyze(
             item_ref: record
                 .and_then(|r| r.placement(*number))
                 .map(|p| p.item.clone()),
+            variant: one_variant(set, *number),
             n,
             n_answered,
             blank_rate: blank as f64 / responded as f64,
@@ -906,6 +915,25 @@ fn reliability(coded: &[Vec<f64>], totals: &[f64], p_values: &[f64], rpbs: &[f64
 /// # Returns
 ///
 /// The letters that appear on full-credit responses.
+/// The single variant every row for one question names, if they agree.
+///
+/// Disagreement is not an error, it is a fact about the administration: a course
+/// that draws distractors per form has two option sets under one question
+/// number, and no pooled statistic describes both. Returning `None` is what
+/// keeps the per-variant records from claiming otherwise.
+fn one_variant(set: &ResponseSet, number: u32) -> Option<String> {
+    let mut seen: Option<&str> = None;
+    for row in set.rows.iter().filter(|r| r.item_number == number) {
+        let variant = row.variant.as_deref()?;
+        match seen {
+            None => seen = Some(variant),
+            Some(first) if first == variant => {}
+            Some(_) => return None,
+        }
+    }
+    seen.map(str::to_string)
+}
+
 fn infer_key(rows: &[&crate::responses::Response]) -> Vec<String> {
     let mut out: BTreeSet<String> = BTreeSet::new();
     for r in rows {
@@ -1001,6 +1029,7 @@ mod tests {
             item_number: number,
             item_ref: None,
             item_version: None,
+            variant: None,
             selected: if letter.is_empty() {
                 vec![]
             } else {