feat(coverage): n_units_collected separates sampling from real zeros (#36)

provenance$coverage's n_units_reporting is category-conditional: it
counts governments with rows for the SPECIFIC requested category, which
conflates two different things -- a government never collected that
year (sampling), and one collected but genuinely spending nothing in
that category (a real zero). FY2012 Georgia Police is the motivating
case from the issue: a complete census year reads as a 69% "response
rate" because most of the gap is cities that contract policing to the
county sheriff, not non-response.

Adds a second counter, n_units_collected: how many of the caller's
expected cohort appear in the corpus that year for ANY category.
n_units_collected / n_units_expected is the true collection rate;
n_units_reporting / n_units_collected is category participation among
collected units. cog_geographic_rollup() and cog_peer_compare() both
carry it; cog_explain() prints it alongside n_units_reporting.

Two real bugs caught and fixed while finishing this (both against the
already-written, previously-uncommitted draft):

- .coverage_table()'s candidate list for the collection query was
  derived from the category-filtered result rows, not the caller's
  full expected cohort. A government with zero rows in the requested
  category across every requested year never appears in that result,
  so it was silently excluded from n_units_collected too -- collapsing
  the new counter back to the old, broken one for exactly the
  governments it exists to count. Fixed by threading an explicit
  `expected_ids` (all_govids / peer_govids) through instead.
- The collection query hardcoded long_view = "spending_long_harmonized",
  which does not exist on a corpus with schema_version < 5 (R/basis.R
  resolves basis = "raw" there; R/views.R only registers the
  harmonized views on v5+). cog_geographic_rollup()/cog_peer_compare()
  would hard-error on a corpus vintage the package otherwise explicitly
  supports. Fixed by deriving long_view from the basis cog_spending()
  actually resolved (prov$basis) via the existing .select_long_view()
  helper, matching how every other basis-aware query in the package
  already does this.

Also: cog_explain()'s general "complete census only in years ending in
2 or 7" footnote was gated on the OLD counter's absence, making it
permanently unreachable now that both callers always supply the new
one -- ungated it, since the explanation is orthogonal to which
counter set is present. Dropped a dead conditional branch, fixed two
stale roxygen blocks in R/peers.R/R/rollup.R still describing the old
two-counter model, fixed the same staleness in README.md, and switched
two `uscogdata:::` self-references to the package's own convention of
calling internal helpers unqualified.

1101 tests pass (2 skipped live-corpus), including new direct
regression tests for both bugs above (one exercising a government
collected-but-absent from a category result, one running the full
rollup/peer-compare path against a doctored schema_version 4 corpus).

Reviewed by an independent code-reviewer pass (1 HIGH, 1 MEDIUM, 3 LOW
-- all addressed above).

Closes #36.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

R/coverage.R

@@ -84,22 +84,92 @@
 #' `n_units_reporting = 0`, which is precisely the disclosure a silently
 #' missing year fails to make.
 #'
-#' `n_units_reporting` describes the result the caller actually received, so
-#' under `coverage = "consistent"` it reports the balanced count. `is_census_year`
-#' is a statement about the SURVEY CALENDAR, never a claim of completeness:
-#' FY1967 is a census year in which only 97 of Wisconsin's 608 cities report.
-#' `n_units_reporting` is the number that tells the truth.
+#' Three counters are returned, each answering a different question:
+#'
+#'   * `n_units_expected` -- the universe the caller named (govids passed in,
+#'     or peers for cog_peer_compare). "How many governments did you ask
+#'     about?"
+#'   * `n_units_collected` -- how many of those appear in the corpus at all
+#'     that year, in ANY category. This is a statement about survey collection,
+#'     independent of what was asked for: "of the governments you named, how
+#'     many did Census actually collect data from this year?" It separates
+#'     sampling (not collected) from real zeros (collected but spends nothing
+#'     in your category).
+#'   * `n_units_reporting` -- how many of those appear with rows for the
+#'     SPECIFIC category you requested. This is always <= n_units_collected:
+#'     a government can be collected but have no rows for "Police" because it
+#'     contracts policing to the county sheriff, not because it wasn't
+#'     surveyed.
+#'
+#' `n_units_reporting` therefore conflates two very different things: a unit
+#' that was not collected (sampling) and a unit that was collected but spends
+#' nothing in that category. The ratio n_units_collected / n_units_expected is
+#' the true collection rate; n_units_reporting / n_units_collected measures
+#' category participation among collected units.
+#'
+#' `is_census_year` is a statement about the SURVEY CALENDAR, never a claim of
+#' completeness: FY1967 is a census year in which only 97 of Wisconsin's 608
+#' cities report. The counters are what tell the truth.
+#'
+#' @param con Active DuckDB connection (used to look up n_units_collected).
+#' @param long_view The verb's own long view, used for the collection query;
+#'   NULL skips the lookup and leaves n_units_collected as NA_integer_.
+#' @param expected_ids The full EXPECTED cohort (govids the caller named),
+#'   used as the candidate list for the collection query. Required alongside
+#'   `con`/`long_view` for a correct count -- see the note below on why it
+#'   must not be derived from `result`/`rows`. `NULL`, or non-`NULL` but
+#'   empty after dropping `NA`/`""` entries, skips the lookup and leaves
+#'   n_units_collected as NA_integer_.
 #' @noRd
 .coverage_table <- function(result, years, n_expected,
-                            id_col = "canonical_govid", rows = NULL) {
+                            id_col = "canonical_govid", rows = NULL,
+                            con = NULL, long_view = NULL,
+                            expected_ids = NULL) {
   years <- sort(unique(as.integer(years)))
   src <- if (is.null(rows)) result else rows
   reporting <- vapply(years, function(y) {
     ids <- src[[id_col]][as.integer(src$year) == y]
     length(unique(ids[!is.na(ids)]))
   }, integer(1))
+
+  # n_units_collected: count EXPECTED cohort members present in the corpus
+  # for ANY category that year, not just the requested one. This separates
+  # sampling (not collected at all) from real zeros (collected but no rows
+  # for this category). Only computed when a connection, long_view, AND
+  # expected_ids are all provided; otherwise NA_integer_.
+  #
+  # The candidate list MUST be expected_ids, not derived from `result`/
+  # `rows`: a government with zero rows in the requested category across
+  # EVERY requested year never appears in `result` at all, so deriving
+  # candidates from it would silently exclude exactly the "collected but
+  # real zero" governments this counter exists to count -- collapsing
+  # n_units_collected back to n_units_reporting for precisely the case #36
+  # was filed over.
+  if (!is.null(con) && !is.null(long_view) && length(expected_ids) > 0L) {
+    cohort_chr <- .sql_lit_chr(unique(expected_ids[!is.na(expected_ids) &
+                                                       nzchar(expected_ids)]))
+    years_lit <- paste(years, collapse = ",")
+    collected_q <- sprintf(
+      "SELECT year, COUNT(DISTINCT canonical_govid) AS n
+       FROM %s
+       WHERE canonical_govid IN (%s)
+         AND year IN (%s)
+       GROUP BY year",
+      long_view, cohort_chr, years_lit
+    )
+    collected_df <- DBI::dbGetQuery(con, collected_q)
+    collected_map <- setNames(collected_df$n, as.integer(collected_df$year))
+    collected <- vapply(years, function(y) {
+      val <- collected_map[as.character(y)]
+      if (is.na(val)) 0L else as.integer(val)
+    }, integer(1))
+  } else {
+    collected <- rep(NA_integer_, length(years))
+  }
+
   tibble::tibble(
     year              = years,
+    n_units_collected = collected,
     n_units_reporting = as.integer(reporting),
     n_units_expected  = rep(as.integer(n_expected), length(years)),
     is_census_year    = .is_census_year(years)

R/explain.R

@@ -166,12 +166,30 @@ cog_explain <- function(result, format = c("print", "list")) {
     cli::cli_h2("Reporting coverage")
     cli::cli_text("Mode: {prov$coverage_mode %||% 'all'}")
     cov <- prov$coverage
-    cli::cli_ul(sprintf(
-      "%d: %d of %d units reporting (%.0f%%) -- %s year",
-      cov$year, cov$n_units_reporting, cov$n_units_expected,
-      100 * cov$n_units_reporting / pmax(cov$n_units_expected, 1L),
-      ifelse(cov$is_census_year, "census", "sample")
-    ))
+    has_collected <- "n_units_collected" %in% names(cov)
+    if (has_collected) {
+      # Three counters: collected separates sampling from real zeros;
+      # reporting is category-conditional and never a response rate.
+      cli::cli_ul(sprintf(
+        "%d: %d of %d units collected, %d reporting in this category -- %s year",
+        cov$year,
+        cov$n_units_collected,
+        cov$n_units_expected,
+        cov$n_units_reporting,
+        ifelse(cov$is_census_year, "census", "sample")
+      ))
+    } else {
+      cli::cli_ul(sprintf(
+        "%d: %d of %d units reporting (%.0f%%) -- %s year",
+        cov$year, cov$n_units_reporting, cov$n_units_expected,
+        100 * cov$n_units_reporting / pmax(cov$n_units_expected, 1L),
+        ifelse(cov$is_census_year, "census", "sample")
+      ))
+    }
+    # Explains what the per-row "-- sample year" tag means, regardless of
+    # which branch above rendered it -- not gated on has_collected, which
+    # would make this permanently unreachable now that both real callers
+    # (cog_geographic_rollup(), cog_peer_compare()) always supply it.
     if (any(!cov$is_census_year)) {
       cli::cli_text(
         "Note: the Census of Governments is a complete census only in years ending in 2 or 7; every other year is a sample."

R/peers.R

@@ -195,11 +195,13 @@ cog_find_peers <- function(target_govid,
 #'     a balanced panel.
 #'
 #'   Regardless of mode, `provenance$coverage` always carries per-year
-#'   `n_units_reporting`, `n_units_expected` and `is_census_year`, and
-#'   `provenance$coverage_mode` records the mode. `is_census_year` is a
-#'   statement about the **survey calendar**, never a claim of completeness:
-#'   FY1967 is a census year in which only 97 of Wisconsin's 608 cities
-#'   report. `n_units_reporting` is the number that tells the truth.
+#'   `n_units_expected`, `n_units_collected`, `n_units_reporting` and
+#'   `is_census_year`, and `provenance$coverage_mode` records the mode.
+#'   `is_census_year` is a statement about the **survey calendar**, never a
+#'   claim of completeness: FY1967 is a census year in which only 97 of
+#'   Wisconsin's 608 cities report. `n_units_reporting` is
+#'   category-conditional and is not a response rate on its own -- see
+#'   "Reading `coverage`" below for what each counter answers.
 #'
 #'   The comparison target is exempt from `"consistent"` balancing -- it is the
 #'   subject of the comparison, not a member of the cohort -- and the
@@ -241,14 +243,23 @@ cog_find_peers <- function(target_govid,
 #'     summarise(p50 = quantile(total, 0.5, na.rm = TRUE))
 #'   ```
 #' @section Reading `coverage`:
-#' `provenance$coverage` reports `n_units_reporting` against
-#' `n_units_expected` per year. **`n_units_reporting` is category-conditional:
-#' it counts cohort members with rows for the category you asked for, not
-#' cohort members collected that year.** A government that was surveyed and
-#' genuinely spends nothing in that category is indistinguishable here from one
-#' that was never surveyed.
+#' `provenance$coverage` carries three per-year counters:
+#'
+#'   * `n_units_expected` -- how many governments you asked about.
+#'   * `n_units_collected` -- how many of those appear in the corpus at all
+#'     that year (in ANY category), separating sampling from real zeros.
+#'   * `n_units_reporting` -- how many have rows for the SPECIFIC category you
+#'     requested. This is always <= n_units_collected: a government can be
+#'     collected but have no rows for "Police" because it contracts policing
+#'     to the county sheriff, not because it wasn't surveyed.
+#'
+#' **`n_units_reporting` is category-conditional** and therefore **not a
+#' response rate**: `n_units_reporting / n_units_expected` conflates sampling
+#' (never collected) with real zeros (collected but spends nothing in your
+#' category). Use `n_units_collected / n_units_expected` for the true
+#' collection rate, and `n_units_reporting / n_units_collected` for category
+#' participation among collected units.
 #'
-#' The ratio is therefore **not a response rate** and must not be used as one.
 #' In FY2022 — a complete census year — Georgia reports 393 of 567 cities for
 #' `category = "Police"`; the 174-city gap is overwhelmingly cities that
 #' contract policing to the county sheriff, not non-response.
@@ -325,10 +336,20 @@ cog_peer_compare <- function(target_govid, peers, category, years,
   # Counted over PEER rows only, against the cohort size: "3 of your 15 peers
   # reported in FY2019". Including the target would inflate every count by one
   # and make a cohort that has entirely stopped reporting look non-empty.
+  # n_units_collected is looked up against the spending long view matching
+  # whatever basis cog_spending() actually resolved above (prov$basis) --
+  # NOT hardcoded to spending_long_harmonized, which does not exist on a
+  # corpus with schema_version < 5 (R/basis.R resolves basis = "raw" there,
+  # and only *_long, not *_long_harmonized, is registered; see R/views.R).
+  # The con comes from .ensure_session() already called inside cog_spending().
+  con <- .ensure_session()
   prov$coverage_mode <- coverage
   prov$coverage <- .coverage_table(
     out, years, length(peer_govids),
-    rows = r[r$role == "peer", , drop = FALSE]
+    rows = r[r$role == "peer", , drop = FALSE],
+    con = con,
+    long_view = .select_long_view("spending_annotated", prov$basis),
+    expected_ids = peer_govids
   )
   attr(out, "provenance") <- prov
   out

R/rollup.R

@@ -49,11 +49,13 @@
 #'     a balanced panel.
 #'
 #'   Regardless of mode, `provenance$coverage` always carries per-year
-#'   `n_units_reporting`, `n_units_expected` and `is_census_year`, and
-#'   `provenance$coverage_mode` records the mode. `is_census_year` is a
-#'   statement about the **survey calendar**, never a claim of completeness:
-#'   FY1967 is a census year in which only 97 of Wisconsin's 608 cities
-#'   report. `n_units_reporting` is the number that tells the truth.
+#'   `n_units_expected`, `n_units_collected`, `n_units_reporting` and
+#'   `is_census_year`, and `provenance$coverage_mode` records the mode.
+#'   `is_census_year` is a statement about the **survey calendar**, never a
+#'   claim of completeness: FY1967 is a census year in which only 97 of
+#'   Wisconsin's 608 cities report. `n_units_reporting` is
+#'   category-conditional and is not a response rate on its own -- see
+#'   "Reading `coverage`" below for what each counter answers.
 #' @return Tibble with columns `year`, `layer`, `canonical_govid`, `gov_name`,
 #'   `spend_subtype`, `category`, `amt_nominal`, optional `amt_real` /
 #'   `amt_per_capita_nominal` / `amt_per_capita_real`, optional `pop_source`,
@@ -61,14 +63,23 @@
 #'   `provenance` attribute with `verb = "cog_geographic_rollup"`, `layers`,
 #'   and `rollup$included_govids` / `rollup$excluded_govids`.
 #' @section Reading `coverage`:
-#' `provenance$coverage` reports `n_units_reporting` against
-#' `n_units_expected` per year. **`n_units_reporting` is category-conditional:
-#' it counts governments with rows for the category you asked for, not
-#' governments collected that year.** A government that was surveyed and
-#' genuinely spends nothing in that category is indistinguishable here from one
-#' that was never surveyed.
+#' `provenance$coverage` carries three per-year counters:
+#'
+#'   * `n_units_expected` -- how many governments you asked about.
+#'   * `n_units_collected` -- how many of those appear in the corpus at all
+#'     that year (in ANY category), separating sampling from real zeros.
+#'   * `n_units_reporting` -- how many have rows for the SPECIFIC category you
+#'     requested. This is always <= n_units_collected: a government can be
+#'     collected but have no rows for "Police" because it contracts policing
+#'     to the county sheriff, not because it wasn't surveyed.
+#'
+#' **`n_units_reporting` is category-conditional** and therefore **not a
+#' response rate**: `n_units_reporting / n_units_expected` conflates sampling
+#' (never collected) with real zeros (collected but spends nothing in your
+#' category). Use `n_units_collected / n_units_expected` for the true
+#' collection rate, and `n_units_reporting / n_units_collected` for category
+#' participation among collected units.
 #'
-#' The ratio is therefore **not a response rate** and must not be used as one.
 #' In FY2022 — a complete census year — Georgia reports 393 of 567 cities for
 #' `category = "Police"`; the 174-city gap is overwhelmingly cities that
 #' contract policing to the county sheriff, not non-response.
@@ -137,8 +148,20 @@ cog_geographic_rollup <- function(govids, category, years,
   # n_units_expected is the universe the CALLER named -- the govids passed in
   # -- not the national universe. That is what makes the ratio meaningful:
   # "597 of the 608 Wisconsin cities you asked about reported in FY2012".
+  # n_units_collected is looked up against the spending long view matching
+  # whatever basis cog_spending() actually resolved above (prov$basis) --
+  # NOT hardcoded to spending_long_harmonized, which does not exist on a
+  # corpus with schema_version < 5 (R/basis.R resolves basis = "raw" there,
+  # and only *_long, not *_long_harmonized, is registered; see R/views.R).
+  # The con comes from .ensure_session() already called inside cog_spending().
+  con <- .ensure_session()
   prov$coverage_mode <- coverage
-  prov$coverage <- .coverage_table(r, years, length(unique(all_govids)))
+  prov$coverage <- .coverage_table(
+    r, years, length(unique(all_govids)),
+    con = con,
+    long_view = .select_long_view("spending_annotated", prov$basis),
+    expected_ids = all_govids
+  )
   attr(r, "provenance") <- prov
 
   r