WifiHealthMonitor: improve regression detection

Current SW regression detection uses the following logic

isRegression = recentFailureRate >= highThr && previousFailureRate <= lowThr;

This logic works well when the network starts with a low failure value (e.g., 0~5%) and then increase to a high value (e.g., 20%).

However, this may not work well when the network starts with a high value (e.g. 20%) and then failure rate further increases to a higher value (e.g., 40%). This case is possible, e.g., enterprise network.

To make it work in the above case, change the regression detection logic to

isRegression = recentFailureRate / previousFailureRate >= failureRateRatioMinThr
&& recentFailureCnt >= failureCntMinThr

The 2nd check  is  to ensure the detection confidence level is high when minConnectionAttempt is small.

In addition, use Laplace's rule of succession to calculate the
probability of failure p = (f + 1) / (n + 2) which has a better
estimation especially for small n.

Bug: 152096939
Test: atest com.android.server.wifi
Change-Id: I037c1a4f80f8094580209765ec31f933cb5d50fa

3 files changed, 122 insertions, 71 deletions

diff --git a/tests/wifitests/src/com/android/server/wifi/DeviceConfigFacadeTest.java b/tests/wifitests/src/com/android/server/wifi/DeviceConfigFacadeTest.java
index 1737231aa..d6a05c7a3 100644
--- a/tests/wifitests/src/com/android/server/wifi/DeviceConfigFacadeTest.java
+++ b/tests/wifitests/src/com/android/server/wifi/DeviceConfigFacadeTest.java
@@ -138,28 +138,30 @@ public class DeviceConfigFacadeTest extends WifiBaseTest {
                 mDeviceConfigFacade.getRxPktPerSecondThr());
         assertEquals(DeviceConfigFacade.DEFAULT_CONNECTION_FAILURE_HIGH_THR_PERCENT,
                 mDeviceConfigFacade.getConnectionFailureHighThrPercent());
-        assertEquals(DeviceConfigFacade.DEFAULT_CONNECTION_FAILURE_LOW_THR_PERCENT,
-                mDeviceConfigFacade.getConnectionFailureLowThrPercent());
+        assertEquals(DeviceConfigFacade.DEFAULT_CONNECTION_FAILURE_COUNT_MIN,
+                mDeviceConfigFacade.getConnectionFailureCountMin());
         assertEquals(DeviceConfigFacade.DEFAULT_ASSOC_REJECTION_HIGH_THR_PERCENT,
                 mDeviceConfigFacade.getAssocRejectionHighThrPercent());
-        assertEquals(DeviceConfigFacade.DEFAULT_ASSOC_REJECTION_LOW_THR_PERCENT,
-                mDeviceConfigFacade.getAssocRejectionLowThrPercent());
+        assertEquals(DeviceConfigFacade.DEFAULT_ASSOC_REJECTION_COUNT_MIN,
+                mDeviceConfigFacade.getAssocRejectionCountMin());
         assertEquals(DeviceConfigFacade.DEFAULT_ASSOC_TIMEOUT_HIGH_THR_PERCENT,
                 mDeviceConfigFacade.getAssocTimeoutHighThrPercent());
-        assertEquals(DeviceConfigFacade.DEFAULT_ASSOC_TIMEOUT_LOW_THR_PERCENT,
-                mDeviceConfigFacade.getAssocTimeoutLowThrPercent());
+        assertEquals(DeviceConfigFacade.DEFAULT_ASSOC_TIMEOUT_COUNT_MIN,
+                mDeviceConfigFacade.getAssocTimeoutCountMin());
         assertEquals(DeviceConfigFacade.DEFAULT_AUTH_FAILURE_HIGH_THR_PERCENT,
                 mDeviceConfigFacade.getAuthFailureHighThrPercent());
-        assertEquals(DeviceConfigFacade.DEFAULT_AUTH_FAILURE_LOW_THR_PERCENT,
-                mDeviceConfigFacade.getAuthFailureLowThrPercent());
+        assertEquals(DeviceConfigFacade.DEFAULT_AUTH_FAILURE_COUNT_MIN,
+                mDeviceConfigFacade.getAuthFailureCountMin());
         assertEquals(DeviceConfigFacade.DEFAULT_SHORT_CONNECTION_NONLOCAL_HIGH_THR_PERCENT,
                 mDeviceConfigFacade.getShortConnectionNonlocalHighThrPercent());
-        assertEquals(DeviceConfigFacade.DEFAULT_SHORT_CONNECTION_NONLOCAL_LOW_THR_PERCENT,
-                mDeviceConfigFacade.getShortConnectionNonlocalLowThrPercent());
+        assertEquals(DeviceConfigFacade.DEFAULT_SHORT_CONNECTION_NONLOCAL_COUNT_MIN,
+                mDeviceConfigFacade.getShortConnectionNonlocalCountMin());
         assertEquals(DeviceConfigFacade.DEFAULT_DISCONNECTION_NONLOCAL_HIGH_THR_PERCENT,
                 mDeviceConfigFacade.getDisconnectionNonlocalHighThrPercent());
-        assertEquals(DeviceConfigFacade.DEFAULT_DISCONNECTION_NONLOCAL_LOW_THR_PERCENT,
-                mDeviceConfigFacade.getDisconnectionNonlocalLowThrPercent());
+        assertEquals(DeviceConfigFacade.DEFAULT_DISCONNECTION_NONLOCAL_COUNT_MIN,
+                mDeviceConfigFacade.getDisconnectionNonlocalCountMin());
+        assertEquals(DeviceConfigFacade.DEFAULT_HEALTH_MONITOR_RATIO_THR_NUMERATOR,
+                mDeviceConfigFacade.getHealthMonitorRatioThrNumerator());
         assertEquals(DeviceConfigFacade.DEFAULT_HEALTH_MONITOR_MIN_RSSI_THR_DBM,
                 mDeviceConfigFacade.getHealthMonitorMinRssiThrDbm());
         assertEquals(Collections.emptySet(),
@@ -207,28 +209,30 @@ public class DeviceConfigFacadeTest extends WifiBaseTest {
                 anyInt())).thenReturn(5);
         when(DeviceConfig.getInt(anyString(), eq("connection_failure_high_thr_percent"),
                 anyInt())).thenReturn(31);
-        when(DeviceConfig.getInt(anyString(), eq("connection_failure_low_thr_percent"),
-                anyInt())).thenReturn(3);
+        when(DeviceConfig.getInt(anyString(), eq("connection_failure_count_min"),
+                anyInt())).thenReturn(4);
         when(DeviceConfig.getInt(anyString(), eq("assoc_rejection_high_thr_percent"),
                 anyInt())).thenReturn(10);
-        when(DeviceConfig.getInt(anyString(), eq("assoc_rejection_low_thr_percent"),
-                anyInt())).thenReturn(2);
+        when(DeviceConfig.getInt(anyString(), eq("assoc_rejection_count_min"),
+                anyInt())).thenReturn(5);
         when(DeviceConfig.getInt(anyString(), eq("assoc_timeout_high_thr_percent"),
                 anyInt())).thenReturn(12);
-        when(DeviceConfig.getInt(anyString(), eq("assoc_timeout_low_thr_percent"),
-                anyInt())).thenReturn(3);
+        when(DeviceConfig.getInt(anyString(), eq("assoc_timeout_count_min"),
+                anyInt())).thenReturn(6);
         when(DeviceConfig.getInt(anyString(), eq("auth_failure_high_thr_percent"),
                 anyInt())).thenReturn(11);
-        when(DeviceConfig.getInt(anyString(), eq("auth_failure_low_thr_percent"),
-                anyInt())).thenReturn(2);
+        when(DeviceConfig.getInt(anyString(), eq("auth_failure_count_min"),
+                anyInt())).thenReturn(7);
         when(DeviceConfig.getInt(anyString(), eq("short_connection_nonlocal_high_thr_percent"),
                 anyInt())).thenReturn(8);
-        when(DeviceConfig.getInt(anyString(), eq("short_connection_nonlocal_low_thr_percent"),
-                anyInt())).thenReturn(1);
+        when(DeviceConfig.getInt(anyString(), eq("short_connection_nonlocal_count_min"),
+                anyInt())).thenReturn(8);
         when(DeviceConfig.getInt(anyString(), eq("disconnection_nonlocal_high_thr_percent"),
                 anyInt())).thenReturn(12);
-        when(DeviceConfig.getInt(anyString(), eq("disconnection_nonlocal_low_thr_percent"),
-                anyInt())).thenReturn(2);
+        when(DeviceConfig.getInt(anyString(), eq("disconnection_nonlocal_count_min"),
+                anyInt())).thenReturn(9);
+        when(DeviceConfig.getInt(anyString(), eq("health_monitor_ratio_thr_numerator"),
+                anyInt())).thenReturn(3);
         when(DeviceConfig.getInt(anyString(), eq("health_monitor_min_rssi_thr_dbm"),
                 anyInt())).thenReturn(-67);
         String testSsidList = "ssid_1,ssid_2";
@@ -263,17 +267,18 @@ public class DeviceConfigFacadeTest extends WifiBaseTest {
         assertEquals(10, mDeviceConfigFacade.getTxPktPerSecondThr());
         assertEquals(5, mDeviceConfigFacade.getRxPktPerSecondThr());
         assertEquals(31, mDeviceConfigFacade.getConnectionFailureHighThrPercent());
-        assertEquals(3, mDeviceConfigFacade.getConnectionFailureLowThrPercent());
+        assertEquals(4, mDeviceConfigFacade.getConnectionFailureCountMin());
         assertEquals(10, mDeviceConfigFacade.getAssocRejectionHighThrPercent());
-        assertEquals(2, mDeviceConfigFacade.getAssocRejectionLowThrPercent());
+        assertEquals(5, mDeviceConfigFacade.getAssocRejectionCountMin());
         assertEquals(12, mDeviceConfigFacade.getAssocTimeoutHighThrPercent());
-        assertEquals(3, mDeviceConfigFacade.getAssocTimeoutLowThrPercent());
+        assertEquals(6, mDeviceConfigFacade.getAssocTimeoutCountMin());
         assertEquals(11, mDeviceConfigFacade.getAuthFailureHighThrPercent());
-        assertEquals(2, mDeviceConfigFacade.getAuthFailureLowThrPercent());
+        assertEquals(7, mDeviceConfigFacade.getAuthFailureCountMin());
         assertEquals(8, mDeviceConfigFacade.getShortConnectionNonlocalHighThrPercent());
-        assertEquals(1, mDeviceConfigFacade.getShortConnectionNonlocalLowThrPercent());
+        assertEquals(8, mDeviceConfigFacade.getShortConnectionNonlocalCountMin());
         assertEquals(12, mDeviceConfigFacade.getDisconnectionNonlocalHighThrPercent());
-        assertEquals(2, mDeviceConfigFacade.getDisconnectionNonlocalLowThrPercent());
+        assertEquals(9, mDeviceConfigFacade.getDisconnectionNonlocalCountMin());
+        assertEquals(3, mDeviceConfigFacade.getHealthMonitorRatioThrNumerator());
         assertEquals(-67, mDeviceConfigFacade.getHealthMonitorMinRssiThrDbm());
         assertEquals(testSsidSet, mDeviceConfigFacade.getRandomizationFlakySsidHotlist());
         assertEquals(testSsidSet,
diff --git a/tests/wifitests/src/com/android/server/wifi/WifiHealthMonitorTest.java b/tests/wifitests/src/com/android/server/wifi/WifiHealthMonitorTest.java
index 89de82675..3342f93a1 100644
--- a/tests/wifitests/src/com/android/server/wifi/WifiHealthMonitorTest.java
+++ b/tests/wifitests/src/com/android/server/wifi/WifiHealthMonitorTest.java
@@ -169,34 +169,34 @@ public class WifiHealthMonitorTest extends WifiBaseTest {
         when(mWifiInjector.getWifiScanner()).thenReturn(mWifiScanner);
         when(mWifiNative.getDriverVersion()).thenReturn(mDriverVersion);
         when(mWifiNative.getFirmwareVersion()).thenReturn(mFirmwareVersion);
-        when(mDeviceConfigFacade.getHealthMonitorMinRssiThrDbm()).thenReturn(
-                DeviceConfigFacade.DEFAULT_HEALTH_MONITOR_MIN_RSSI_THR_DBM);
         when(mDeviceConfigFacade.getConnectionFailureHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_CONNECTION_FAILURE_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getConnectionFailureLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_CONNECTION_FAILURE_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getConnectionFailureCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_CONNECTION_FAILURE_COUNT_MIN);
         when(mDeviceConfigFacade.getAssocRejectionHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_ASSOC_REJECTION_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getAssocRejectionLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_ASSOC_REJECTION_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getAssocRejectionCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_ASSOC_REJECTION_COUNT_MIN);
         when(mDeviceConfigFacade.getAssocTimeoutHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_ASSOC_TIMEOUT_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getAssocTimeoutLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_ASSOC_TIMEOUT_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getAssocTimeoutCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_ASSOC_TIMEOUT_COUNT_MIN);
         when(mDeviceConfigFacade.getAuthFailureHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_AUTH_FAILURE_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getAuthFailureLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_AUTH_FAILURE_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getAuthFailureCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_AUTH_FAILURE_COUNT_MIN);
         when(mDeviceConfigFacade.getShortConnectionNonlocalHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_SHORT_CONNECTION_NONLOCAL_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getShortConnectionNonlocalLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_SHORT_CONNECTION_NONLOCAL_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getShortConnectionNonlocalCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_SHORT_CONNECTION_NONLOCAL_COUNT_MIN);
         when(mDeviceConfigFacade.getDisconnectionNonlocalHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_DISCONNECTION_NONLOCAL_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getDisconnectionNonlocalLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_DISCONNECTION_NONLOCAL_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getDisconnectionNonlocalCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_DISCONNECTION_NONLOCAL_COUNT_MIN);
         when(mDeviceConfigFacade.getHealthMonitorMinRssiThrDbm()).thenReturn(
                 DeviceConfigFacade.DEFAULT_HEALTH_MONITOR_MIN_RSSI_THR_DBM);
+        when(mDeviceConfigFacade.getHealthMonitorRatioThrNumerator()).thenReturn(
+                DeviceConfigFacade.DEFAULT_HEALTH_MONITOR_RATIO_THR_NUMERATOR);
         when(mDeviceConfigFacade.getHealthMonitorMinNumConnectionAttempt()).thenReturn(
                 DeviceConfigFacade.DEFAULT_HEALTH_MONITOR_MIN_NUM_CONNECTION_ATTEMPT);
         mWifiHealthMonitor = new WifiHealthMonitor(mContext, mWifiInjector, mClock,
diff --git a/tests/wifitests/src/com/android/server/wifi/WifiScoreCardTest.java b/tests/wifitests/src/com/android/server/wifi/WifiScoreCardTest.java
index f52837572..e98b03acf 100644
--- a/tests/wifitests/src/com/android/server/wifi/WifiScoreCardTest.java
+++ b/tests/wifitests/src/com/android/server/wifi/WifiScoreCardTest.java
@@ -126,30 +126,32 @@ public class WifiScoreCardTest extends WifiBaseTest {
         mWifiScoreCard.mPersistentHistograms = true; // TODO - remove when ready
         when(mDeviceConfigFacade.getConnectionFailureHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_CONNECTION_FAILURE_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getConnectionFailureLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_CONNECTION_FAILURE_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getConnectionFailureCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_CONNECTION_FAILURE_COUNT_MIN);
         when(mDeviceConfigFacade.getAssocRejectionHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_ASSOC_REJECTION_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getAssocRejectionLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_ASSOC_REJECTION_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getAssocRejectionCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_ASSOC_REJECTION_COUNT_MIN);
         when(mDeviceConfigFacade.getAssocTimeoutHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_ASSOC_TIMEOUT_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getAssocTimeoutLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_ASSOC_TIMEOUT_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getAssocTimeoutCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_ASSOC_TIMEOUT_COUNT_MIN);
         when(mDeviceConfigFacade.getAuthFailureHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_AUTH_FAILURE_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getAuthFailureLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_AUTH_FAILURE_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getAuthFailureCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_AUTH_FAILURE_COUNT_MIN);
         when(mDeviceConfigFacade.getShortConnectionNonlocalHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_SHORT_CONNECTION_NONLOCAL_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getShortConnectionNonlocalLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_SHORT_CONNECTION_NONLOCAL_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getShortConnectionNonlocalCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_SHORT_CONNECTION_NONLOCAL_COUNT_MIN);
         when(mDeviceConfigFacade.getDisconnectionNonlocalHighThrPercent()).thenReturn(
                 DeviceConfigFacade.DEFAULT_DISCONNECTION_NONLOCAL_HIGH_THR_PERCENT);
-        when(mDeviceConfigFacade.getDisconnectionNonlocalLowThrPercent()).thenReturn(
-                DeviceConfigFacade.DEFAULT_DISCONNECTION_NONLOCAL_LOW_THR_PERCENT);
+        when(mDeviceConfigFacade.getDisconnectionNonlocalCountMin()).thenReturn(
+                DeviceConfigFacade.DEFAULT_DISCONNECTION_NONLOCAL_COUNT_MIN);
         when(mDeviceConfigFacade.getHealthMonitorMinRssiThrDbm()).thenReturn(
                 DeviceConfigFacade.DEFAULT_HEALTH_MONITOR_MIN_RSSI_THR_DBM);
+        when(mDeviceConfigFacade.getHealthMonitorRatioThrNumerator()).thenReturn(
+                DeviceConfigFacade.DEFAULT_HEALTH_MONITOR_RATIO_THR_NUMERATOR);
         when(mDeviceConfigFacade.getHealthMonitorMinNumConnectionAttempt()).thenReturn(
                 DeviceConfigFacade.DEFAULT_HEALTH_MONITOR_MIN_NUM_CONNECTION_ATTEMPT);
         mWifiScoreCard.enableVerboseLogging(true);
@@ -1107,22 +1109,32 @@ public class WifiScoreCardTest extends WifiBaseTest {
     }
 
     /**
-     * Run a few days with good connection, followed by a SW build change which results
+     * Run a few days with mostly good connection and some failures,
+     * followed by a SW build change which results
      * in performance regression. Check if the regression is detected properly.
      */
     @Test
     public void testRegressionAfterSwBuildChange() throws Exception {
         PerNetwork perNetwork = mWifiScoreCard.lookupNetwork(mWifiInfo.getSSID());
-        int numGoodConnectionDays = 5;
+        int numGoodConnectionDays = 4;
         for (int i = 0; i < numGoodConnectionDays; i++) {
             makeRecentStatsWithGoodConnection();
             perNetwork.updateAfterDailyDetection();
         }
-
-        perNetwork.updateAfterSwBuildChange();
+        // Extra day with mixed failures
         makeRecentStatsWithShortConnection();
         makeRecentStatsWithAssocTimeOut();
         makeRecentStatsWithAuthFailure();
+        perNetwork.updateAfterDailyDetection();
+
+        perNetwork.updateAfterSwBuildChange();
+        // Add >2x failures after the SW build change
+        int numBadConnectionDays = 4;
+        for (int i = 0; i < numBadConnectionDays; i++) {
+            makeRecentStatsWithShortConnection();
+            makeRecentStatsWithAssocTimeOut();
+            makeRecentStatsWithAuthFailure();
+        }
 
         FailureStats statsDec = new FailureStats();
         FailureStats statsInc = new FailureStats();
@@ -1135,25 +1147,59 @@ public class WifiScoreCardTest extends WifiBaseTest {
     }
 
     /**
+     * Check regression detection is missed due to low failure count
+     */
+    @Test
+    public void testMissRegressionDetectionDuetoLowFailureCnt() throws Exception {
+        PerNetwork perNetwork = mWifiScoreCard.lookupNetwork(mWifiInfo.getSSID());
+        int numGoodConnectionDays = 1;
+        for (int i = 0; i < numGoodConnectionDays; i++) {
+            makeRecentStatsWithGoodConnection();
+            perNetwork.updateAfterDailyDetection();
+        }
+
+        perNetwork.updateAfterSwBuildChange();
+        makeRecentStatsWithGoodConnection();
+        // Add a small number of failures for each failure type after the SW build change
+        for (int i = 0; i < mDeviceConfigFacade.getAuthFailureCountMin() - 1; i++) {
+            makeShortConnectionExample();
+            makeAssocTimeOutExample();
+            makeAuthFailureExample();
+        }
+
+        FailureStats statsDec = new FailureStats();
+        FailureStats statsInc = new FailureStats();
+        FailureStats statsHigh = new FailureStats();
+        int detectionFlag = perNetwork.dailyDetection(statsDec, statsInc, statsHigh);
+        assertEquals(WifiScoreCard.SUFFICIENT_RECENT_PREV_STATS, detectionFlag);
+        checkStatsDeltaExample(statsDec, 0);
+        checkStatsDeltaExample(statsInc, 0);
+        checkStatsDeltaExample(statsHigh, 0);
+    }
+
+    /**
      * Run a few days with bad connections, followed by a SW build change which results
      * in performance improvement. Check if the improvement is detected properly.
      */
     @Test
     public void testImprovementAfterSwBuildChange() throws Exception {
         PerNetwork perNetwork = mWifiScoreCard.lookupNetwork(mWifiInfo.getSSID());
-        makeRecentStatsWithGoodConnection(); // Day 1
-        perNetwork.updateAfterDailyDetection();
-        makeRecentStatsWithAssocTimeOut();   // Day 2
-        perNetwork.updateAfterDailyDetection();
-        makeRecentStatsWithAuthFailure();    // Day 3
-        perNetwork.updateAfterDailyDetection();
-        makeRecentStatsWithShortConnection(); // Day 4
-        perNetwork.updateAfterDailyDetection();
-        makeRecentStatsWithShortConnection(); // Day 5
-        perNetwork.updateAfterDailyDetection();
+        for (int i = 0; i < 2; i++) {
+            makeRecentStatsWithShortConnection();
+            makeRecentStatsWithAssocTimeOut();
+            makeRecentStatsWithAuthFailure();
+            perNetwork.updateAfterDailyDetection();
+        }
 
         perNetwork.updateAfterSwBuildChange();
-        makeRecentStatsWithGoodConnection(); // Day 6
+        // Add <50% failures after the SW build change
+        int numGoodConnectionDays = 4;
+        for (int i = 0; i < numGoodConnectionDays; i++) {
+            makeRecentStatsWithGoodConnection();
+        }
+        makeRecentStatsWithShortConnection();
+        makeRecentStatsWithAssocTimeOut();
+        makeRecentStatsWithAuthFailure();
 
         FailureStats statsDec = new FailureStats();
         FailureStats statsInc = new FailureStats();