# Duplication analysis

    library(scrutiny)

You can use scrutiny to detect duplicate values in any dataset. Duplicates can go a long way in assessing the reliability of published research.

This vignette walks you through scrutiny’s tools for detecting, counting, and summarizing duplicates. It uses the `pigs4` dataset as a simple example:

    pigs4
    #> # A tibble: 5 × 3
    #>   snout tail  wings
    #>   <chr> <chr> <chr>
    #> 1 4.73  6.88  6.09
    #> 2 8.13  7.33  8.27
    #> 3 4.22  5.17  4.40
    #> 4 4.22  7.57  5.92
    #> 5 5.17  8.13  5.17

## Frequency tabulation with `duplicate_count()`

A good first step is to create a frequency table. To do so, use `duplicate_count()`:

    pigs4 %>%
      duplicate_count()
    #> # A tibble: 11 × 4
    #>    value frequency locations          locations_n
    #>    <chr>     <int> <chr>                    <int>
    #>  1 5.17          3 snout, tail, wings           3
    #>  2 4.22          2 snout                        1
    #>  3 8.13          2 snout, tail                  2
    #>  4 4.73          1 snout                        1
    #>  5 6.88          1 tail                         1
    #>  6 7.33          1 tail                         1
    #>  7 7.57          1 tail                         1
    #>  8 4.40          1 wings                        1
    #>  9 5.92          1 wings                        1
    #> 10 6.09          1 wings                        1
    #> 11 8.27          1 wings                        1

It returns a tibble (data frame) that lists each unique `value`. The tibble is ordered by the `frequency` of values in the input data frame, so the values that appear most often are at the top. The `locations` are the names of all the columns in which a given value appears. They are counted by `locations_n`.

For example, `5.17` is the most frequent value in `pigs4`. It appears 3 times (`frequency`), namely in the `snout`, `tail`, and `wings` columns; so `locations_n` is also `3`. The next most frequent value is `4.22` which appears twice, but both of these instances are in the `snout` column, so `locations_n` is `1`.

Run `audit()` after `duplicate_count()` to get summary statistics for the two numeric columns, `frequency` and `locations_n`:

    pigs4 %>%
        duplicate_count() %>%
        audit()
    #> # A tibble: 2 × 8
    #>   term         mean    sd median   min   max na_count na_rate
    #>   <chr>       <dbl> <dbl>  <dbl> <dbl> <dbl>    <dbl>   <dbl>
    #> 1 frequency    1.36 0.674      1     1     3        0       0
    #> 2 locations_n  1.27 0.647      1     1     3        0       0

## Counting by column pair with `duplicate_count_colpair()`

Sometimes, a sequence of data may be repeated in multiple columns. `duplicate_count_colpair()` helps find such cases:

    pigs4 %>%
      duplicate_count_colpair()
    #> # A tibble: 3 × 7
    #>   x     y     count total_x total_y rate_x rate_y
    #>   <chr> <chr> <int>   <int>   <int>  <dbl>  <dbl>
    #> 1 snout tail      2       5       5    0.4    0.4
    #> 2 snout wings     1       5       5    0.2    0.2
    #> 3 tail  wings     1       5       5    0.2    0.2

`x` and `y` represent all combinations of columns in `pigs4`. The `count` is the number of values that appear in both respective columns. `total_x` and `total_y` are the numbers of non-missing values in the original columns listed under `x` and `y`. Similarly, `rate_x` is the rate of `x` values that also appear in `y`, and `rate_y` is the rate of `y` values that also appear in `x`. If there are no missing values, `total_x` is the same as `total_y`, and `rate_x` is the same as `rate_y`.

Here, `snout` and `tail` are the column pair with the most overlap: 2 out of 5 values are the same, a duplication rate of 0.4.

Again, you can call `audit()` for summary statistics:

    pigs4 %>%
      duplicate_count_colpair() %>%
      audit()
    #> # A tibble: 5 × 8
    #>   term     mean    sd median   min   max na_count na_rate
    #>   <chr>   <dbl> <dbl>  <dbl> <dbl> <dbl>    <dbl>   <dbl>
    #> 1 count   1.33  0.577    1     1     2          0       0
    #> 2 total_x 5     0        5     5     5          0       0
    #> 3 total_y 5     0        5     5     5          0       0
    #> 4 rate_x  0.267 0.115    0.2   0.2   0.4        0       0
    #> 5 rate_y  0.267 0.115    0.2   0.2   0.4        0       0

## Counting by observation with `duplicate_tally()`

Unlike the other two functions, `duplicate_tally()` largely preserves the structure of the original data frame. It only adds a column ending on `_n` next to each original column. The new columns count how often the values to their left appear in the data frame as a whole:

    pigs4 %>%
        duplicate_tally()
    #> # A tibble: 5 × 6
    #>   snout snout_n tail  tail_n wings wings_n
    #>   <chr>   <int> <chr>  <int> <chr>   <int>
    #> 1 4.73        1 6.88       1 6.09        1
    #> 2 8.13        2 7.33       1 8.27        1
    #> 3 4.22        2 5.17       3 4.40        1
    #> 4 4.22        2 7.57       1 5.92        1
    #> 5 5.17        3 8.13       2 5.17        3

In `snout`, for example, `4.22` appears twice, so its entries in `snout_n` are `2`. But likewise, `8.13` appears in both `snout` and `tail`, so both observations are marked `2` in the `_n` columns.

When following up `duplicate_tally()` with `audit()`, it shows summary statistics for each `_n` column. The last row summarizes all of these columns together:

    pigs4 %>%
        duplicate_tally() %>%
        audit()
    #> # A tibble: 4 × 8
    #>   term     mean    sd median   min   max na_count na_rate
    #>   <chr>   <dbl> <dbl>  <dbl> <dbl> <dbl>    <dbl>   <dbl>
    #> 1 snout_n  2    0.707      2     1     3        0       0
    #> 2 tail_n   1.6  0.894      1     1     3        0       0
    #> 3 wings_n  1.4  0.894      1     1     3        0       0
    #> 4 .total   1.67 0.816      1     1     3        0       0