diff --git a/chrony.texi b/chrony.texi
index bee73f5..d25421f 100644
--- a/chrony.texi
+++ b/chrony.texi
@@ -1828,7 +1828,7 @@ An example line (which actually appears as a single line in the file)
 from the measurements log file is shown below.
 
 @example
-1998-07-22 05:40:50 158.152.1.76    N  8 1111 11 1111 10 10  1 \
+2010-12-22 05:40:50 158.152.1.76    N  8 1111 111 1111 10 10 1.0 \
    -4.966e-03  2.296e-01  1.577e-05  1.615e-01  7.446e-03
 @end example
 
@@ -1837,7 +1837,7 @@ values from the example line above) :
 
 @enumerate 1
 @item
-Date [1998-07-22]
+Date [2010-12-22]
 @item
 Hour:Minute:Second [05:40:50].  Note that the date/time pair is
 expressed in UTC, not the local time zone.
@@ -1853,8 +1853,8 @@ Stratum of remote computer. [2]
 @item
 RFC1305 tests 1 through 4 (1=pass, 0=fail) [1111]
 @item
-Tests for maximum delay and maximum delay ratio, against user defined
-parameters (1=pass, 0=fail) [11]
+Tests for maximum delay, maximum delay ratio and maximum delay dev ratio,
+against defined parameters (1=pass, 0=fail) [111]
 @item
 RFC1305 tests 5 through 8 (1=pass, 0=fail) [1111]
 @item
@@ -2595,6 +2595,12 @@ measurements that it has buffered.  If a measurement has a round trip
 delay that is greater than the maxdelayratio times the minimum delay, it
 will be rejected.
 
+@item maxdelaydevratio
+If a measurement has ratio of the increase in round-trip delay from
+the minimum delay amongst the previous measurements to the standard
+deviation of the previous measurements that is greater than
+maxdelaydevratio, it will be rejected. The default is 10.0.
+
 @item presend
 If the timing measurements being made by @code{chronyd} are the only
 network data passing between two computers, you may find that some
diff --git a/client.c b/client.c
index eaf92ab..67456e4 100644
--- a/client.c
+++ b/client.c
@@ -963,6 +963,11 @@ process_cmd_add_server_or_peer(CMD_Request *msg, char *line)
         break;
       }
 
+      if (data.params.max_delay_dev_ratio != SRC_DEFAULT_MAXDELAYDEVRATIO) {
+        fprintf(stderr, "Option maxdelaydevratio not supported\n");
+        break;
+      }
+
       msg->data.ntp_source.port = htonl((unsigned long) data.port);
       UTI_IPHostToNetwork(&data.ip_addr, &msg->data.ntp_source.ip_addr);
       msg->data.ntp_source.minpoll = htonl(data.params.minpoll);
@@ -998,6 +1003,9 @@ process_cmd_add_server_or_peer(CMD_Request *msg, char *line)
     case CPS_BadPresend:
       fprintf(stderr, "Unreadable presend value\n");
       break;
+    case CPS_BadMaxdelaydevratio:
+      fprintf(stderr, "Unreadable max delay dev ratio value\n");
+      break;
     case CPS_BadMaxdelayratio:
       fprintf(stderr, "Unreadable max delay ratio value\n");
       break;
diff --git a/cmdmon.c b/cmdmon.c
index 884c623..3d5df74 100644
--- a/cmdmon.c
+++ b/cmdmon.c
@@ -1271,6 +1271,7 @@ handle_add_source(NTP_Source_Type type, CMD_Request *rx_message, CMD_Reply *tx_m
  /* not transmitted in cmdmon protocol yet */
   params.min_stratum = SRC_DEFAULT_MINSTRATUM;       
   params.poll_target = SRC_DEFAULT_POLLTARGET;
+  params.max_delay_dev_ratio = SRC_DEFAULT_MAXDELAYDEVRATIO;
 
   status = NSR_AddSource(&rem_addr, type, &params);
   switch (status) {
diff --git a/cmdparse.c b/cmdparse.c
index be45989..983c334 100644
--- a/cmdparse.c
+++ b/cmdparse.c
@@ -56,6 +56,7 @@ CPS_ParseNTPSourceAdd(const char *line, CPS_NTP_Source *src)
   src->params.authkey = INACTIVE_AUTHKEY;
   src->params.max_delay = SRC_DEFAULT_MAXDELAY;
   src->params.max_delay_ratio = SRC_DEFAULT_MAXDELAYRATIO;
+  src->params.max_delay_dev_ratio = SRC_DEFAULT_MAXDELAYDEVRATIO;
   src->params.online = 1;
   src->params.auto_offline = 0;
   src->params.iburst = 0;
@@ -123,6 +124,14 @@ CPS_ParseNTPSourceAdd(const char *line, CPS_NTP_Source *src)
           } else {
             line += n;
           }
+        } else if (!strncasecmp(cmd, "maxdelaydevratio", 16)) {
+          if (sscanf(line, "%lf%n", &src->params.max_delay_dev_ratio, &n) != 1) {
+            result = CPS_BadMaxdelaydevratio;
+            ok = 0;
+            done = 1;
+          } else {
+            line += n;
+          }
           /* This MUST come before the following one ! */
         } else if (!strncasecmp(cmd, "maxdelayratio", 13)) {
           if (sscanf(line, "%lf%n", &src->params.max_delay_ratio, &n) != 1) {
diff --git a/cmdparse.h b/cmdparse.h
index a6353bd..0f41cfd 100644
--- a/cmdparse.h
+++ b/cmdparse.h
@@ -42,6 +42,7 @@ typedef enum {
   CPS_BadMinpoll,
   CPS_BadMaxpoll,
   CPS_BadPresend,
+  CPS_BadMaxdelaydevratio,
   CPS_BadMaxdelayratio,
   CPS_BadMaxdelay,
   CPS_BadKey,
diff --git a/conf.c b/conf.c
index f6fd9b2..b64c4bc 100644
--- a/conf.c
+++ b/conf.c
@@ -391,6 +391,9 @@ parse_source(const char *line, NTP_Source_Type type)
     case CPS_BadPresend:
       LOG(LOGS_WARN, LOGF_Configure, "Unreadable presend value at line %d", line_number);
       break;
+    case CPS_BadMaxdelaydevratio:
+      LOG(LOGS_WARN, LOGF_Configure, "Unreadable max delay dev ratio value at line %d", line_number);
+      break;
     case CPS_BadMaxdelayratio:
       LOG(LOGS_WARN, LOGF_Configure, "Unreadable max delay ratio value at line %d", line_number);
       break;
diff --git a/ntp_core.c b/ntp_core.c
index 8306ad2..4211594 100644
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -119,6 +119,8 @@ struct NCR_Instance_Record {
                                    min_delay_in_register that we can
                                    tolerate.  */
 
+  double max_delay_dev_ratio;   /* Maximum ratio of increase in delay / stddev */
+
   int do_auth;                  /* Flag indicating whether we
                                    authenticate packets we send to
                                    this machine (if it's serving us or
@@ -213,7 +215,7 @@ void
 NCR_Initialise(void)
 {
   logfileid = CNF_GetLogMeasurements() ? LOG_FileOpen("measurements",
-      "   Date (UTC) Time     IP Address   L St 1234 ab 5678 LP RP Score Offset     Peer del. Peer disp. Root del.  Root disp.")
+      "   Date (UTC) Time     IP Address   L St 1234 abc 5678 LP RP Score Offset     Peer del. Peer disp. Root del.  Root disp.")
     : -1;
 
   access_auth_table = ADF_CreateTable();
@@ -284,6 +286,7 @@ NCR_GetInstance(NTP_Remote_Address *remote_addr, NTP_Source_Type type, SourcePar
 
   result->max_delay = params->max_delay;
   result->max_delay_ratio = params->max_delay_ratio;
+  result->max_delay_dev_ratio = params->max_delay_dev_ratio;
 
   result->tx_count = 0;
 
@@ -777,7 +780,7 @@ receive_packet(NTP_Packet *message, struct timeval *now, double now_err, NCR_Ins
 
   int test1, test2, test3, test4, test5, test6, test7, test8;
 
-  int test4a, test4b;
+  int test4a, test4b, test4c;
 
   /* In the words of section 3.4.4 of RFC1305, valid_data means
      that the NTP protocol association with the peer/server is
@@ -941,6 +944,18 @@ receive_packet(NTP_Packet *message, struct timeval *now, double now_err, NCR_Ins
     test4b = 1; /* Success */
   }
 
+  /* Test 4c (additional to RFC1305) requires that the ratio of the
+     increase in delay from the minimum one in the stats data register to
+     the standard deviation of the offsets in the register is less than an
+     administrator-defined value or the difference between measured offset
+     and predicted offset is larger than the increase in delay */
+  if (!SRC_IsGoodSample(inst->source, -theta, delta, inst->max_delay_dev_ratio,
+        LCL_GetMaxClockError(), &sample_time)) {
+    test4c = 0; /* Failed */
+  } else {
+    test4c = 1; /* Success */
+  }
+
   /* Test 5 relates to authentication. */
   if (inst->do_auth) {
     if (do_auth) {
@@ -1011,7 +1026,7 @@ receive_packet(NTP_Packet *message, struct timeval *now, double now_err, NCR_Ins
 
   valid_kod = test1 && test2 && test5;
 
-  valid_data = test1 && test2 && test3 && test4 && test4a && test4b;
+  valid_data = test1 && test2 && test3 && test4 && test4a && test4b && test4c;
   valid_header = test5 && test6 && test7 && test8;
 
   root_delay = pkt_root_delay + delta;
@@ -1240,13 +1255,13 @@ receive_packet(NTP_Packet *message, struct timeval *now, double now_err, NCR_Ins
 
   /* Do measurement logging */
   if (logfileid != -1) {
-    LOG_FileWrite(logfileid, "%s %-15s %1c %2d %1d%1d%1d%1d %1d%1d %1d%1d%1d%1d %2d %2d %4.2f %10.3e %10.3e %10.3e %10.3e %10.3e",
+    LOG_FileWrite(logfileid, "%s %-15s %1c %2d %1d%1d%1d%1d %1d%1d%1d %1d%1d%1d%1d %2d %2d %4.2f %10.3e %10.3e %10.3e %10.3e %10.3e",
             UTI_TimeToLogForm(sample_time.tv_sec),
             UTI_IPToString(&inst->remote_addr.ip_addr),
             sync_stats[pkt_leap],
             message->stratum,
             test1, test2, test3, test4,
-            test4a, test4b,
+            test4a, test4b, test4c,
             test5, test6, test7, test8,
             inst->local_poll, inst->remote_poll,
             inst->poll_score,
diff --git a/sources.c b/sources.c
index 38706ca..20c0855 100644
--- a/sources.c
+++ b/sources.c
@@ -794,6 +794,16 @@ SRC_MinRoundTripDelay(SRC_Instance inst)
   return SST_MinRoundTripDelay(inst->stats);
 }
 
+/* ================================================== */
+
+int
+SRC_IsGoodSample(SRC_Instance inst, double offset, double delay,
+   double max_delay_dev_ratio, double clock_error, struct timeval *when)
+{
+  return SST_IsGoodSample(inst->stats, offset, delay, max_delay_dev_ratio,
+      clock_error, when);
+}
+
 /* ================================================== */
 /* This routine is registered as a callback with the local clock
    module, to be called whenever the local clock changes frequency or
diff --git a/sources.h b/sources.h
index d89ce59..b731191 100644
--- a/sources.h
+++ b/sources.h
@@ -145,6 +145,10 @@ extern double SRC_PredictOffset(SRC_Instance inst, struct timeval *when);
    currently held in the register */
 extern double SRC_MinRoundTripDelay(SRC_Instance inst);
 
+/* This routine determines if a new sample is good enough that it should be
+   accumulated */
+extern int SRC_IsGoodSample(SRC_Instance inst, double offset, double delay, double max_delay_dev_ratio, double clock_error, struct timeval *when);
+
 extern void SRC_DumpSources(void);
 
 extern void SRC_ReloadSources(void);
diff --git a/sourcestats.c b/sourcestats.c
index c149db9..522abd0 100644
--- a/sourcestats.c
+++ b/sourcestats.c
@@ -693,6 +693,45 @@ SST_MinRoundTripDelay(SST_Stats inst)
   return inst->peer_delays[inst->min_delay_sample];
 }
 
+/* ================================================== */
+
+int
+SST_IsGoodSample(SST_Stats inst, double offset, double delay,
+    double max_delay_dev_ratio, double clock_error, struct timeval *when)
+{
+  double elapsed, allowed_increase, delay_increase;
+
+  if (inst->n_samples < 3)
+    return 1;
+
+  UTI_DiffTimevalsToDouble(&elapsed, when, &inst->offset_time);
+
+  /* Require that the ratio of the increase in delay from the minimum to the
+     standard deviation is less than max_delay_dev_ratio. In the allowed
+     increase in delay include also skew and clock_error. */
+    
+  allowed_increase = sqrt(inst->variance) * max_delay_dev_ratio +
+    elapsed * (inst->skew + clock_error);
+  delay_increase = (delay - SST_MinRoundTripDelay(inst)) / 2.0;
+
+  if (delay_increase < allowed_increase)
+    return 1;
+
+  offset -= inst->estimated_offset + elapsed * inst->estimated_frequency;
+
+  /* Before we decide to drop the sample, make sure the difference between
+     measured offset and predicted offset is not significantly larger than
+     the increase in delay */
+  if (fabs(offset) - delay_increase > allowed_increase)
+    return 1;
+
+#if 0
+  LOG(LOGS_INFO, LOGF_SourceStats, "bad sample: offset=%f delay=%f incr_delay=%f allowed=%f", offset, delay, allowed_increase, delay_increase);
+#endif
+
+  return 0;
+}
+
 /* ================================================== */
 /* This is used to save the register to a file, so that we can reload
    it after restarting the daemon */
diff --git a/sourcestats.h b/sourcestats.h
index e47287f..27908c8 100644
--- a/sourcestats.h
+++ b/sourcestats.h
@@ -137,6 +137,11 @@ extern double SST_PredictOffset(SST_Stats inst, struct timeval *when);
 /* Find the minimum round trip delay in the register */
 extern double SST_MinRoundTripDelay(SST_Stats inst);
 
+/* This routine determines if a new sample is good enough that it should be
+   accumulated */
+extern int SST_IsGoodSample(SST_Stats inst, double offset, double delay,
+   double max_delay_dev_ratio, double clock_error, struct timeval *when);
+
 extern void SST_SaveToFile(SST_Stats inst, FILE *out);
 
 extern int SST_LoadFromFile(SST_Stats inst, FILE *in);
diff --git a/srcparams.h b/srcparams.h
index 58395cb..0cab90a 100644
--- a/srcparams.h
+++ b/srcparams.h
@@ -45,6 +45,7 @@ typedef struct {
   unsigned long authkey;
   double max_delay;
   double max_delay_ratio;
+  double max_delay_dev_ratio;
   SRC_SelectOption sel_option;
 } SourceParameters;
 
@@ -54,6 +55,7 @@ typedef struct {
 #define SRC_DEFAULT_PRESEND_MINPOLL 0
 #define SRC_DEFAULT_MAXDELAY 16.0
 #define SRC_DEFAULT_MAXDELAYRATIO 16384.0
+#define SRC_DEFAULT_MAXDELAYDEVRATIO 10.0
 #define SRC_DEFAULT_MINSTRATUM 0
 #define SRC_DEFAULT_POLLTARGET 6
 #define INACTIVE_AUTHKEY 0UL