diff --git a/zerver/lib/statistics.py b/zerver/lib/statistics.py
index 57b5ba7d8e..e53b1654ba 100644
--- a/zerver/lib/statistics.py
+++ b/zerver/lib/statistics.py
@@ -5,13 +5,16 @@ from __future__ import division
 from zerver.models import UserProfile, UserActivity, UserActivityInterval, Message
 
 from django.utils.timezone import utc
+from typing import Any
+
+from datetime import datetime, timedelta
 
-from datetime import timedelta
 from itertools import chain
 from six.moves import range
 import six
 
 def median(data):
+    # type: (List[float]) -> float
     data = sorted(data)
 
     size = len(data)
@@ -27,29 +30,35 @@ users_who_sent_query = Message.objects.select_related("sender") \
         .exclude(sending_client__name__contains="API")
 
 def active_users():
+    # type: () -> List[UserProfile]
     # Return a list of active users we want to count towards various
     # statistics.
     return UserProfile.objects.filter(is_bot=False, is_active=True).select_related()
 
 def users_who_sent_between(begin, end):
+    # type: (datetime, datetime) -> Set[int]
     sender_objs = users_who_sent_query.filter(pub_date__gt=begin, pub_date__lt=end) \
         .values("sender__id")
     return set(s["sender__id"] for s in sender_objs)
 
 def users_who_sent_ever():
+    # type: () -> Set[int]
     return set(s["sender__id"] for s in users_who_sent_query.values("sender__id"))
 
 def active_users_to_measure():
+    # type: () -> List[UserProfile]
     senders = users_who_sent_ever()
     return [u for u in active_users() if u.id in senders]
 
 def active_users_who_sent_between(begin, end):
+    # type: (datetime, datetime) -> List[UserProfile]
     senders = users_who_sent_between(begin, end)
     return [u for u in active_users() if u.id in senders]
 
 # Return the amount of Zulip usage for this user between the two
 # given dates
 def seconds_usage_between(user_profile, begin, end):
+    # type: (UserProfile, datetime, datetime) -> timedelta
     intervals = UserActivityInterval.objects.filter(user_profile=user_profile, end__gte=begin, start__lte=end)
     duration = timedelta(0)
     for interval in intervals:
@@ -60,6 +69,7 @@ def seconds_usage_between(user_profile, begin, end):
 
 # Return a list of how many seconds each user has been engaging with the app on a given day
 def seconds_active_during_day(day):
+    # type: (datetime) -> List[float]
     begin_day = day.replace(hour=0, minute=0, second=0, microsecond=0, tzinfo=utc)
     end_day = day.replace(hour=23, minute=59, second=59, microsecond=0, tzinfo=utc)
     active_users = active_users_to_measure()
@@ -67,6 +77,7 @@ def seconds_active_during_day(day):
     return [seconds_usage_between(user, begin_day, end_day).total_seconds() for user in active_users]
 
 def users_active_nosend_during_day(day):
+    # type: (datetime) -> List[UserProfile]
     begin_day = day.replace(hour=0, minute=0, second=0, microsecond=0, tzinfo=utc)
     end_day = day.replace(hour=23, minute=59, second=59, microsecond=0, tzinfo=utc)
     active_users = active_users_to_measure()
@@ -82,6 +93,7 @@ def users_active_nosend_during_day(day):
     return [u for u in today_users if not u.id in today_senders]
 
 def calculate_stats(data, all_users):
+    # type: (List[float], List[UserProfile]) -> Dict[str, Any]
     if len(data) == 0:
         return {"# data points": 0}
 
@@ -97,6 +109,7 @@ def calculate_stats(data, all_users):
 
 # Return an info dict {mean: , median} containing the mean/median seconds users were active on a given day
 def activity_averages_during_day(day):
+    # type: (datetime) -> Dict[str, Any]
     users_to_measure = active_users_to_measure()
     seconds_active = seconds_active_during_day(day)
     return calculate_stats(seconds_active, all_users=users_to_measure)
@@ -104,6 +117,7 @@ def activity_averages_during_day(day):
 # Returns an info dict {mean: , median} with engagement numbers for all users according
 # to active_users_to_measure.
 def activity_averages_between(begin, end, by_day=True):
+    # type: (datetime, datetime, bool) -> Dict[str, Any]
     seconds_active = {}
     users_to_measure = active_users_to_measure()
     for i in range((end - begin).days):