The tsibble package extends the tidyverse to temporal data. Built
on top of the tibble, a
tsibble (or tbl_ts) is a data- and model-oriented object.
Compared to the conventional time series objects in R, for example
ts, zoo, and xts, the tsibble
preserves time indices as the essential data column and makes
heterogeneous data structures possible. Beyond the tibble-like
representation, key comprised of single or multiple
variables is introduced to uniquely identify observational units over
time (index). The tsibble package aims at managing
temporal data and getting analysis done in a fluent workflow.
Contextual semantics: index and key
tsibble() creates a tsibble object, and
as_tsibble() is an S3 method to coerce other objects to a
tsibble. An object that a vector/matrix underlies, such as
ts and mts, can be automated to a tsibble
using as_tsibble() without any specification. If it is a
tibble or data frame, as_tsibble() requires a little more
setup in order to declare the index and key variables.
library(dplyr)
library(lubridate)
library(tsibble)
weather <- nycflights13::weather %>%
select(origin, time_hour, temp, humid, precip)
weather
#> # A tibble: 26,115 × 5
#> origin time_hour temp humid precip
#> <chr> <dttm> <dbl> <dbl> <dbl>
#> 1 EWR 2013-01-01 01:00:00 39.0 59.4 0
#> 2 EWR 2013-01-01 02:00:00 39.0 61.6 0
#> 3 EWR 2013-01-01 03:00:00 39.0 64.4 0
#> 4 EWR 2013-01-01 04:00:00 39.9 62.2 0
#> 5 EWR 2013-01-01 05:00:00 39.0 64.4 0
#> # ℹ 26,110 more rowsThe weather data included in the package
nycflights13 contains the hourly meteorological records
(such as temperature, humid and precipitation) over the year of 2013 at
three stations (i.e. JFK, LGA and EWR) in New York City. Since the
time_hour is the only column involving the timestamps,
as_tsibble() defaults it to the index variable;
alternatively, the index can be specified by the argument
index = time_hour to disable the verbose message.
Except for index, a tsibble requires “key”, which defines subjects or
individuals measured over time. In this example, the origin
variable is the identifier, which is passed to the argument
key in as_tsibble(). Each observation
should be uniquely identified by index and key in a valid
tsibble. Others—temp, humid and
precip—are referred to as measured variables. When creating
a tsibble, the key will be sorted first, followed by arranging time from
past to recent.
weather_tsbl <- as_tsibble(weather, key = origin)
#> Using `time_hour` as index variable.
weather_tsbl
#> # A tsibble: 26,115 x 5 [1h] <America/New_York>
#> # Key: origin [3]
#> origin time_hour temp humid precip
#> <chr> <dttm> <dbl> <dbl> <dbl>
#> 1 EWR 2013-01-01 01:00:00 39.0 59.4 0
#> 2 EWR 2013-01-01 02:00:00 39.0 61.6 0
#> 3 EWR 2013-01-01 03:00:00 39.0 64.4 0
#> 4 EWR 2013-01-01 04:00:00 39.9 62.2 0
#> 5 EWR 2013-01-01 05:00:00 39.0 64.4 0
#> # ℹ 26,110 more rowsAn interval is automatically obtained based on the corresponding time representation:
-
integer/numeric/ordered: either “unit” or “year” (Y) -
yearquarter/yearqtr: “quarter” (Q) -
yearmonth/yearmon: “month” (M) -
yearweek: “week” (W) -
Date: “day” (D) -
difftime: “week” (W), “day” (D), “hour” (h), “minute” (m), “second” (s) -
POSIXct/hms: “hour” (h), “minute” (m), “second” (s), “millisecond” (us), “microsecond” (ms) -
nanotime: “nanosecond” (ns)
That is, a tsibble of monthly intervals expects the
yearmonth/yearmon class in the index column.
Neither Date nor POSIXct gives a monthly
tsibble.
The print display is data-centric and contextually informative, such
as data dimension, time interval, and the number of time-based units.
Above displays the weather_tsbl its one-hour interval
([1h]) and the origin [3] as the key along
with three time series in the table.
Data pipeline
This tidy data representation most naturally supports thinking of
operations on the data as building blocks, forming part of a “data
pipeline” in time-based context. Users who are familiar with tidyverse
would find it easier to perform common temporal analysis tasks. For
example, index_by() is the counterpart of
group_by() in temporal context, but it only groups the time
index. index_by() + summarise() is used to
summarise daily highs and lows at each station. As a result, the index
is updated to the date with one-day interval from the index
time_hour; two new variables are created and computed for
daily maximum and minimum temperatures.
weather_tsbl %>%
group_by_key() %>%
index_by(date = ~ as_date(.)) %>%
summarise(
temp_high = max(temp, na.rm = TRUE),
temp_low = min(temp, na.rm = TRUE)
)
#> # A tsibble: 1,092 x 4 [1D]
#> # Key: origin [3]
#> origin date temp_high temp_low
#> <chr> <date> <dbl> <dbl>
#> 1 EWR 2013-01-01 41 28.0
#> 2 EWR 2013-01-02 34.0 24.1
#> 3 EWR 2013-01-03 34.0 26.1
#> 4 EWR 2013-01-04 39.9 28.9
#> 5 EWR 2013-01-05 44.1 32
#> # ℹ 1,087 more rowsIrregular time interval
Note that the tsibble handles regularly-spaced temporal data well,
from seconds to years based on its time representation (see
?tsibble). The option regular, by default, is
set to TRUE in as_tsibble(). Specify
regular to FALSE to create a tsibble for the
data collected at irregular time interval. Below shows the scheduled
date time of the flights in New York:
flights <- nycflights13::flights %>%
mutate(sched_dep_datetime =
make_datetime(year, month, day, hour, minute, tz = "America/New_York"))The key contains columns carrier and flight
to identify observational units over time, from a passenger’s point of
view. With regular = FALSE, it turns to an
irregularly-spaced tsibble, where [!] highlights the
irregularity.
flights_tsbl <- flights %>%
as_tsibble(
key = c(carrier, flight),
index = sched_dep_datetime,
regular = FALSE
)
flights_tsbl
#> # A tsibble: 336,776 x 20 [!] <America/New_York>
#> # Key: carrier, flight [5,725]
#> year month day dep_time sched_dep_time dep_delay arr_time sched_arr_time
#> <int> <int> <int> <int> <int> <dbl> <int> <int>
#> 1 2013 11 3 1531 1540 -9 1653 1725
#> 2 2013 11 4 1539 1540 -1 1712 1725
#> 3 2013 11 5 1548 1540 8 1708 1725
#> 4 2013 11 6 1535 1540 -5 1657 1725
#> 5 2013 11 7 1549 1540 9 1733 1725
#> # ℹ 336,771 more rows
#> # ℹ 12 more variables: arr_delay <dbl>, carrier <chr>, flight <int>,
#> # tailnum <chr>, origin <chr>, dest <chr>, air_time <dbl>, distance <dbl>,
#> # hour <dbl>, minute <dbl>, time_hour <dttm>, sched_dep_datetime <dttm>To regularise an irregular tsibble, it can be achieved with
index_by() + summarise().