Event_ID | Person_ID | Start_Period | End_Period | Event_Type
------------------------------------------------------------
A        | Person1   | 1            | 9          | Assessment
B        | Person2   | 3            | 6          | Activity
C        | Person3   | 7            | 13         | Assessment

Person_ID | Period
----------------------
Person1   | 1
Person1   | 2
Person1   | 3
Person2   | 1
Person2   | 2
Person2   | 3

Person_ID | Period | ActivitiesFreq | AssessmentsFreq
----------------------------------------------
Person1   | 1      | 3              | 5
Person1   | 2      | 3              | 2
Person1   | 3      | 2              | 7
Person2   | 1      | 7              | 2

mutate(PP,SUM(EV$Person_ID==Person_ID,EV$Start_Period<=Period,EV$End_Period>=Period)

Warning messages:
1: In mutate_impl(.data, dots) :
  is.na() applied to non-(list or vector) of type 'NULL'
2: In mutate_impl(.data, dots) :
  longer object length is not a multiple of shorter object length
3: In mutate_impl(.data, dots) :
  longer object length is not a multiple of shorter object length