Stata FAQ
How can I detect duplicate observations?

This Stata FAQ shows how to check if a dataset has duplicate observations.  There are two methods available for this task.  The first example will use commands available in base Stata.  The second example will use a user-written program.  This user-written command is nice because it creates a variable that captures all the information needed to replicate any deleted observations. 

Example 1

This example uses the High School and Beyond dataset, which has no duplicate observations.  Therefore, we add five duplicate observations to the data, and then use the duplicates command to detect which observations are repeated.  Also, to evaluate the sensitivity of the command, we change a value of one of the duplicate observations.  The rationale for changing a value is to mimic what may happen in practice; we often search for "duplicate" cases that are not identically entered into the dataset.  In the dataset, the variable id is the unique case identifier.

To add the duplicate observations, we sort the data by id, then duplicate the first five observations (id = 1 to 5).  This leads to 195 unique and 5 duplicated observations in the dataset.  For subject id =1, all of her values are duplicated except for her math score; one duplicate score is set to 84.

use http://www.ats.ucla.edu/stat/stata/notes/hsb2, clear
(highschool and beyond (200 cases))

keep id female ses math read write
sort id
expand 2 if _n <=5
(5 observations created)

sort id
replace math = 84 if _n==1
(1 real change made)

list in 1/15, noobs 


  +--------------------------------------------+
  | id   female      ses   read   write   math |
  |--------------------------------------------|
  |  1   female      low     34      44     84 |
  |  1   female      low     34      44     40 |
  |  2   female   middle     39      41     33 |
  |  2   female   middle     39      41     33 |
  |  3     male      low     63      65     48 |
  |--------------------------------------------|
  |  3     male      low     63      65     48 |
  |  4   female      low     44      50     41 |
  |  4   female      low     44      50     41 |
  |  5     male      low     47      40     43 |
  |  5     male      low     47      40     43 |
  |--------------------------------------------|
  |  6   female      low     47      41     46 |
  |  7     male   middle     57      54     59 |
  |  8   female      low     39      44     52 |
  |  9     male   middle     48      49     52 |
  | 10   female   middle     47      54     49 |
  +--------------------------------------------+

We start by running the duplicates report command to see the number of duplicate rows in the dataset.  This is followed by duplicate reports id, which gives the number of replicate rows by the variables specified; in this instance we have just id.  We could have used the duplicates examples command instead of the duplicates report command.  The duplicates examples command lists one example of each duplicated set.

duplicates report

Duplicates in terms of all variables
--------------------------------------
   copies | observations       surplus
----------+---------------------------
        1 |          197             0
        2 |            8             4
--------------------------------------

duplicates report id

Duplicates in terms of id

--------------------------------------
   copies | observations       surplus
----------+---------------------------
        1 |          195             0
        2 |           10             5
--------------------------------------

Clearly, the output from duplicates report and duplicates report id differ.  The duplicates report output shows the number of replicate rows over all variables.  Note that in the duplicate whose value we changed (id=1), the two rows are not technically the same, and this command correctly did not pick them up.  The second command duplicates report id shows that we have 195 unique id values, and five ids (surplus) that appear two times each (copies), which leads to a total of 10 questionable observations based on id.

Next we list duplicate observations with the duplicates list command.

duplicates list, nolabel sepby(id)

  +---------------------------------------------------------+
  | group:   obs:   id   female   ses   read   write   math |
  |---------------------------------------------------------|
  |      1      3    2        1     2     39      41     33 |
  |      1      4    2        1     2     39      41     33 |
  |---------------------------------------------------------|
  |      2      5    3        0     1     63      65     48 |
  |      2      6    3        0     1     63      65     48 |
  |---------------------------------------------------------|
  |      3      7    4        1     1     44      50     41 |
  |      3      8    4        1     1     44      50     41 |
  |---------------------------------------------------------|
  |      4      9    5        0     1     47      40     43 |
  |      4     10    5        0     1     47      40     43 |
  +---------------------------------------------------------+

This duplicates list corresponds to listing those observations with duplicate rows; however, as found with duplicates report, it does not identify the five duplicated ids.  Therefore, we try duplicates list id.

duplicates list id, nolabel sepby(id)

Duplicates in terms of id

  +--------------------+
  | group:   obs:   id |
  |--------------------|
  |      1      1    1 |
  |      1      2    1 |
  |--------------------|
  |      2      3    2 |
  |      2      4    2 |
  |--------------------|
  |      3      5    3 |
  |      3      6    3 |
  |--------------------|
  |      4      7    4 |
  |      4      8    4 |
  |--------------------|
  |      5      9    5 |
  |      5     10    5 |
  +--------------------+

Now we see which five subjects are duplicated; however, the duplicate list only lists the variable specified.  We may want to list the other variables to see which variables are causing the difference between the duplicates list and duplicates list id outputs.  To have an output like that given from duplicates list, we use the duplicates tag command to create a new variable dup_id that assigns a 1 if the id is duplicated, and 0 if it appears once.  Then we list those cases where dup_id is equal to 1.

duplicates tag id, gen(dup_id)

Duplicates in terms of id

list if dup_id == 1, nolabel sepby(id)

  +--------------------------------------------------+
  | id   female   ses   read   write   math   dup_id |
  |--------------------------------------------------|
  |  1        1     1     34      44     84        1 |
  |  1        1     1     34      44     40        1 |
  |--------------------------------------------------|
  |  2        1     2     39      41     33        1 |
  |  2        1     2     39      41     33        1 |
  |--------------------------------------------------|
  |  3        0     1     63      65     48        1 |
  |  3        0     1     63      65     48        1 |
  |--------------------------------------------------|
  |  4        1     1     44      50     41        1 |
  |  4        1     1     44      50     41        1 |
  |--------------------------------------------------|
  |  5        0     1     47      40     43        1 |
  |  5        0     1     47      40     43        1 |
  +--------------------------------------------------+

It is apparent that id = 1 has different values on math scores over the duplicate observations.  From this, it would be advisable to check which score, if either, is the correct one.

Suppose in this instance both scores were incorrect, and the real score was 44.  We correct the scores, and after the correction, the results from duplicates report and duplicates report id should coincide.

replace math = 44 if id==1
(2 real changes made)

Now, we can use the duplicates drop command to drop the duplicate observations.  The command drops all observations except the first occurrence of each group with duplicate observations.  After we run duplicates drop, we check that there are no other duplicate observations.

duplicates drop
Duplicates in terms of all variables
(5 observations deleted)

duplicates report

Duplicates in terms of all variables

--------------------------------------
   copies | observations       surplus
----------+---------------------------
        1 |          200             0
--------------------------------------

It appears that we have gotten rid of the duplicate observations.

Example 2

In Stata, several programs are available to detect the duplicates and can also optionally drop the duplicates.  One of the programs is called dups.  The program dups is not a built-in program in Stata, but can be installed over the internet using findit dups (see How can I use the findit command to search for programs and get additional help? for more information about using findit).  Once dups is installed we can use it right away.  This example uses the following subset of the larger dataset used above with added duplicates.

First we enter the data:

input id female race read
18 0 1 50
22 0 1 42
26 1 2 60
147 1 4 47
171 0 4 60
191 1 4 47
18 0 1 50
18 0 1 50
end

Then we look at them:

list, clean

        id   female   race   read  
  1.    18        0      1     50  
  2.    22        0      1     42  
  3.    26        1      2     60  
  4.   147        1      4     47  
  5.   171        0      4     60  
  6.   191        1      4     47  
  7.    18        0      1     50  
  8.    18        0      1     50  

In our example, we have one group of observations with duplicates consisting of observation number 1, 7 and 8.  That is what we see below.  Now we will use the command dups.  Without any arguments, dups returns information on the number of groups of observations that have duplicates and the number of duplicates in each group.

dups

group by: id female race read

groups formed: 1 containing 3 observations
unique observations: 5

groups of duplicate observations:
  +-----------------+
  | _group   _count |
  |-----------------|
  |      1        3 |
  +-----------------+
We can add a variable list after dups.  For example, in the following example, we add the variable race after dups.  Now dups counts how many duplicate observations in variable race only.  We can see from the list of the data set that there are three groups of observations of race (1, 2 and 4) and two of them have duplicates. That is shown by dups below.
dups race

group by: race

groups formed: 2 containing 7 observations
unique observations: 1

groups of duplicate observations:
  +-----------------+
  | _group   _count |
  |-----------------|
  |      1        4 |
  |      2        3 |
  +-----------------+
By adding the option unique, we also request information on groups that have a single unique observation.  For example,
dups id, unique
group by: id

groups formed: 1 containing 3 observations
unique observations: 5

groups of duplicate observations:
  +-----------------+
  | _group   _count |
  |-----------------|
  |      1        3 |
  +-----------------+

unique observations:
  +-----------------+
  | _group   _count |
  |-----------------|
  |      1        1 |
  |      2        1 |
  |      3        1 |
  |      4        1 |
  |      5        1 |
  +-----------------+

With the option key(varlist), we can request to list the observations.  For example, in the following example, we see the values of id in each group.

dups id, unique key(id)
group by: id

groups formed: 1 containing 3 observations
unique observations: 5

groups of duplicate observations:
  +----------------------+
  | _group   _count   id |
  |----------------------|
  |      1        3   18 |
  +----------------------+

unique observations:
  +-----------------------+
  | _group   _count    id |
  |-----------------------|
  |      1        1    22 |
  |      2        1    26 |
  |      3        1   147 |
  |      4        1   171 |
  |      5        1   191 |
  +-----------------------+

An option called terse can be added to get summary information on duplicates.  For example,

dups id, unique key(id) terse

group by: id

groups formed: 1

total observations:    8
  in duplicates        3
  in unique            5

Now what if we want to drop the duplicates?  We can do so by adding an option called drop.  We do want to warn you that it is always dangerous to delete observations since you may lose your data.  So always do it with caution.  What is nice about dups is that it creates a new variable which has enough information to recover the deleted observations if we change our mind on what we just did.  The default name of the variable is _expand (you can change the name by using the option expand after dups).  Using the variable _expand we can get the deleted observations back by using a command called expand.  See the example below.

dups, drop key(id)

group by: id female race read

groups formed: 1 containing 3 observations
unique observations: 5

groups of duplicate observations:
  +----------------------+
  | _group   _count   id |
  |----------------------|
  |      1        3   18 |
  +----------------------+

(2 observations deleted)

observations remaining: 6

list
     +--------------------------------------+
     |  id   female   race   read   _expand |
     |--------------------------------------|
  1. |  18        0      1     50         3 |
  2. |  22        0      1     42         1 |
  3. |  26        1      2     60         1 |
  4. | 147        1      4     47         1 |
  5. | 171        0      4     60         1 |
     |--------------------------------------|
  6. | 191        1      4     47         1 |
     +--------------------------------------+

If for some reason you wanted to return to a dataset that has duplicates, you can use the expand command, using the _expand variable created by dups to specify the number of duplicates to be made.  Note that this will only recover your original dataset if you detected duplicates based on all variables in your dataset.  If you used only a subset of variables, then you will only be able to accurately recreate the values of those cases (because you have no data on the variables that weren't used to determine duplicates).

expand _expand
(2 observations created)

list
          id     female       race       read    _expand
1.        22          0          1         42          1
2.        26          1          2         60          1
3.       147          1          4         47          1
4.       171          0          4         60          1
5.       191          1          4         47          1
6.        18          0          1         50          3
7.        18          0          1         50          3
8.        18          0          1         50          3

Now we have seen how to detect and drop duplicate observations using the user-written dups command.

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