Log suggestion scores in 10 discrete buckets.

components/ntp_snippets/content_suggestions_metrics.cc

Index: components/ntp_snippets/content_suggestions_metrics.cc
diff --git a/components/ntp_snippets/content_suggestions_metrics.cc b/components/ntp_snippets/content_suggestions_metrics.cc
index 4f61eed1fa9af77c9dbb0f6ea8386caaae14bbf1..58b1dff08b329ef8e5a6a95cb479a64cce7e7f7f 100644
--- a/components/ntp_snippets/content_suggestions_metrics.cc
+++ b/components/ntp_snippets/content_suggestions_metrics.cc
@@ -4,6 +4,7 @@
 
 #include "components/ntp_snippets/content_suggestions_metrics.h"
 
+#include <cmath>
 #include <string>
 #include <type_traits>
 
@@ -25,18 +26,19 @@ const char kHistogramCountOnNtpOpened[] =
     "NewTabPage.ContentSuggestions.CountOnNtpOpened";
 const char kHistogramShown[] = "NewTabPage.ContentSuggestions.Shown";
 const char kHistogramShownAge[] = "NewTabPage.ContentSuggestions.ShownAge";
-const char kHistogramShownScore[] = "NewTabPage.ContentSuggestions.ShownScore";
+const char kHistogramShownScore[] =
+    "NewTabPage.ContentSuggestions.ShownScoreNormalized";
 const char kHistogramOpened[] = "NewTabPage.ContentSuggestions.Opened";
 const char kHistogramOpenedAge[] = "NewTabPage.ContentSuggestions.OpenedAge";
 const char kHistogramOpenedScore[] =
-    "NewTabPage.ContentSuggestions.OpenedScore";
+    "NewTabPage.ContentSuggestions.OpenedScoreNormalized";
 const char kHistogramOpenDisposition[] =
     "NewTabPage.ContentSuggestions.OpenDisposition";
 const char kHistogramMenuOpened[] = "NewTabPage.ContentSuggestions.MenuOpened";
 const char kHistogramMenuOpenedAge[] =
     "NewTabPage.ContentSuggestions.MenuOpenedAge";
 const char kHistogramMenuOpenedScore[] =
-    "NewTabPage.ContentSuggestions.MenuOpenedScore";
+    "NewTabPage.ContentSuggestions.MenuOpenedScoreNormalized";
 const char kHistogramDismissedUnvisited[] =
     "NewTabPage.ContentSuggestions.DismissedUnvisited";
 const char kHistogramDismissedVisited[] =
@@ -168,9 +170,14 @@ void UmaHistogramAge(const std::string& name, const base::TimeDelta& value) {
 // This corresponds to UMA_HISTOGRAM_CUSTOM_COUNTS (with min/max appropriate
 // for the score of suggestions) for use with dynamic histogram names.
 void UmaHistogramScore(const std::string& name, float value) {
-  base::Histogram::FactoryGet(name, 1, 100000, 50,
-                              base::HistogramBase::kUmaTargetedHistogramFlag)
-      ->Add(value);
+  // Scores are typically reported in a range of (0,1]. As UMA does not support
+  // floats, we put them on a discrete scale of [1,10]. We keep the extra bucket
+  // 11 for unexpected over-flows as we want to distinguish them from scores
+  // close to 1. For instance, the discrete value 1 represents score values
+  // within (0.0, 0.1].
+  base::LinearHistogram::FactoryGet(
+      name, 1, 11, 12, base::HistogramBase::kUmaTargetedHistogramFlag)
+      ->Add(ceil(value * 10));

[Alexei Svitkine (slow), 2017/01/12 18:42:04]

Can you use UmaHistogramExactLinear() here?

[tschumann, 2017/01/13 17:57:23]

Yes. That's actually nice! Turns out UmaHistogramLongTimes() already exists
there too (I could use another pair of eyes double-checking they are indeed
identical).

 }
 
 void LogCategoryHistogramEnumeration(const char* base_name,