When you get a new data set, you need to look at the data to get a sense of what it contains and potential problems with it. That’s a key phrase here “looking at the data” - what does that mean?
On the one hand, you can look at the head of the data:
## Sepal.Length Sepal.Width Petal.Length Petal.Width Species
## 1 5.1 3.5 1.4 0.2 setosa
## 2 4.9 3.0 1.4 0.2 setosa
## 3 4.7 3.2 1.3 0.2 setosa
## 4 4.6 3.1 1.5 0.2 setosa
## 5 5.0 3.6 1.4 0.2 setosa
## 6 5.4 3.9 1.7 0.4 setosa
Or you can have a glimpse
at it through
dplyr::glimpse
##
## Attaching package: 'dplyr'
## The following objects are masked from 'package:stats':
##
## filter, lag
## The following objects are masked from 'package:base':
##
## intersect, setdiff, setequal, union
## Rows: 150
## Columns: 5
## $ Sepal.Length <dbl> 5.1, 4.9, 4.7, 4.6, 5.0, 5.4, 4.6, 5.0, 4.4, 4.9, 5.4, 4.…
## $ Sepal.Width <dbl> 3.5, 3.0, 3.2, 3.1, 3.6, 3.9, 3.4, 3.4, 2.9, 3.1, 3.7, 3.…
## $ Petal.Length <dbl> 1.4, 1.4, 1.3, 1.5, 1.4, 1.7, 1.4, 1.5, 1.4, 1.5, 1.5, 1.…
## $ Petal.Width <dbl> 0.2, 0.2, 0.2, 0.2, 0.2, 0.4, 0.3, 0.2, 0.2, 0.1, 0.2, 0.…
## $ Species <fct> setosa, setosa, setosa, setosa, setosa, setosa, setosa, s…
Here we see that we have doubles, and a factor. We get some insight into the data.
But we don’t always have data like the canonical iris dataset. let’s
take a look at some data that might be a bit more typical of “messy”
data using the typical_data
dataset from the
visdat
package.
## Rows: 5,000
## Columns: 9
## $ ID <chr> "0001", "0002", "0003", "0004", "0005", "0006", "0007", "…
## $ Race <fct> Black, Black, Black, Hispanic, NA, White, White, Black, W…
## $ Age <chr> NA, "25", "31", "27", "21", "22", "23", "21", NA, "27", "…
## $ Sex <fct> Male, Male, Female, Female, Female, Female, Female, Femal…
## $ `Height(cm)` <dbl> 175.9, 171.7, 173.5, 172.4, 158.5, 169.5, 163.7, 165.8, 1…
## $ IQ <dbl> 110, 84, 115, 84, 116, 83, 101, 97, 92, 99, 88, 86, NA, 9…
## $ Smokes <lgl> FALSE, TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, FA…
## $ Income <fct> 4334.29, 16682.37, 50402.01, 91442.86, 75266.05, 12209.71…
## $ Died <lgl> FALSE, TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, TR…
Looking at this, you might then ask:
Isn’t it odd that Income is a factor? And Age is a character?
And you might start to wonder what else is different, what else changed?
And it might be a bit unclear where to go from there. Do you plot the
data? Why does my plot look weird? What are these other strange features
in the data? The visdat
package provides visualisations of
an entire dataframe at once. Initially inspired by csv-fingerprint
,
visdat
provides tools to create heatmap-like visualisations
of an entire dataframe. visdat
provides 2 main functions:
vis_dat
and vis_miss
.
vis_dat()
helps explore the data class structure and
missingness:
And the vis_miss
function provides a custom plot for
missing data.
The name visdat
was chosen as it borrows from the idea
of testdat
,
which provides unit testing for your data. In a similar way,
visdat
provides visual tests, the idea being that first you
visualise your data (visdat
), then you run tests from
testdat
, or a package like assertr
, to fix
these errors.
vis_dat
Let’s see what’s inside the dataset airquality
, which
contains information about daily air quality measurements in New York
from May to September 1973. More information about the dataset can be
found with ?airquality
.
The plot above tells us that R reads this dataset as having numeric and
integer values, with some missing data in Ozone
and
Solar.R
. The classes are represented on the legend, and
missing data represented by grey. The column/variable names are listed
on the x axis.
By default, vis_dat
sorts the columns according to the
type of the data in the vectors. You can turn this off by setting
sort_type = FALSE
. This feature is better illustrated using
the typical_data
dataset, created using wakefield and contained
within visdat
.
vis_miss
We can explore the missing data further using
vis_miss
.
Notice that the percentages of missingness are provided in the data.
These are accurate to 1 decimal place. vis_miss
indicates
when there is a very small amount of missing data at <0.1%
missingness.
df_test <- data.frame(x1 = 1:10000,
x2 = rep("A", 10000),
x3 = c(rep(1L, 9999), NA))
vis_miss(df_test)
vis_miss
will also indicate when there is no missing
data at all.
df_test <- data.frame(x1 = 1:10000,
x2 = rep("tidy", 10000),
x3 = rep("data", 10000))
vis_miss(df_test)
Columns can be arranged by columns with most missingness, by setting
sort_miss = TRUE
.
And missingness can be clustered by setting
cluster = TRUE
To further explore the missingness structure in a dataset, I
recommend the naniar
package, which provides more general tools for graphical and numerical
exploration of missing values.
vis_compare
Sometimes you want to see what has changed in your data.
vis_compare()
displays the differences in two dataframes of
the same size. Let’s look at an example.
Let’s make some changes to the chickwts
, and compare
this new dataset.
set.seed(2019-04-03-1107)
chickwts_diff <- chickwts
chickwts_diff[sample(1:nrow(chickwts), 30),sample(1:ncol(chickwts), 2)] <- NA
vis_compare(chickwts_diff, chickwts)
Here the differences are marked in blue.
If you try and compare differences when the dimensions are different, you get an ugly error.
vis_expect
vis_expect
visualises certain conditions or values in
your data. For example, If you are not sure whether to expect values
greater than 25 in your data (airquality), you could write:
vis_expect(airquality, ~.x >= 25)
, and you can see if
there are times where the values in your data are greater than or equal
to 25.
This shows the proportion of times that there are values greater than 25, as well as the missings.
You could also, for example, explore a set of bad strings, or
possible NA values and visualise where they are using
vis_expect(data, ~.x %in% bad_strings)
where
bad_strings
is a character vector containing bad strings
like N A
, N/A
etc.
vis_cor
To make it easy to plot correlations of your data, use
vis_cor
:
Under the hood, vis_cor
is powered by the
cor
function in base R, and takes a character string
indicating which correlation coefficient (or covariance) is to be
computed. One of “pearson” (default), “kendall”, or “spearman”.
You can also specify what to do for the missing data using the
na_action
function, which again borrows from the
cor
methods. This can be “everything”, “all.obs”,
“complete.obs”, “na.or.complete”, or “pairwise.complete.obs” (default),
e.g.:
vis_value
vis_value()
visualises the values of your data on a 0 to
1 scale.
It only works on numeric data:
data input can only contain numeric values, please subset the data to the numeric values you would like. dplyr::select_if(data, is.numeric) can be helpful here!
So you might need to subset the data beforehand like so:
It can be useful to arrange your data before using
vis_value
to explore possible relationships in the
data: