20.2 Stratified Sampling
A stratum is a subset of the population that has at least one common characteristic.
Steps:
- Identify relevant stratums and their representation in the population.
- Randomly sample to select a sufficient number of subjects from each stratum.
Stratified sampling reduces sampling error.
library(dplyr)
# by number of rows
sample_iris <- iris %>%
group_by(Species) %>%
sample_n(5)
sample_iris
#> # A tibble: 15 × 5
#> # Groups: Species [3]
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> <dbl> <dbl> <dbl> <dbl> <fct>
#> 1 4.4 3 1.3 0.2 setosa
#> 2 5.2 3.5 1.5 0.2 setosa
#> 3 5.1 3.8 1.5 0.3 setosa
#> 4 5.2 3.4 1.4 0.2 setosa
#> 5 4.5 2.3 1.3 0.3 setosa
#> 6 5.5 2.5 4 1.3 versicolor
#> 7 7 3.2 4.7 1.4 versicolor
#> 8 6.7 3 5 1.7 versicolor
#> 9 6.1 2.9 4.7 1.4 versicolor
#> 10 5.5 2.4 3.8 1.1 versicolor
#> 11 6.4 2.7 5.3 1.9 virginica
#> 12 6.4 2.8 5.6 2.1 virginica
#> 13 6.4 3.2 5.3 2.3 virginica
#> 14 6.8 3.2 5.9 2.3 virginica
#> 15 7.2 3.6 6.1 2.5 virginica
# by fraction
sample_iris <- iris %>%
group_by(Species) %>%
sample_frac(size = .15)
sample_iris
#> # A tibble: 24 × 5
#> # Groups: Species [3]
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> <dbl> <dbl> <dbl> <dbl> <fct>
#> 1 5.5 4.2 1.4 0.2 setosa
#> 2 5 3 1.6 0.2 setosa
#> 3 5.2 4.1 1.5 0.1 setosa
#> 4 4.6 3.1 1.5 0.2 setosa
#> 5 5.1 3.7 1.5 0.4 setosa
#> 6 4.8 3.4 1.9 0.2 setosa
#> 7 5.1 3.3 1.7 0.5 setosa
#> 8 5.5 3.5 1.3 0.2 setosa
#> 9 5 2.3 3.3 1 versicolor
#> 10 5.6 2.9 3.6 1.3 versicolor
#> # ℹ 14 more rowslibrary(sampler)
# Stratified sample using proportional allocation without replacement
ssamp(df=albania, n=360, strata=qarku, over=0.1)
#> # A tibble: 395 × 45
#> qarku Q_ID bashkia BAS_ID zaz njesiaAdministrative COM_ID qvKod zgjedhes
#> <fct> <int> <fct> <int> <fct> <fct> <int> <fct> <int>
#> 1 Berat 1 Berat 11 ZAZ … "Berat " 1101 "\"3… 558
#> 2 Berat 1 Berat 11 ZAZ … "Berat " 1101 "\"3… 815
#> 3 Berat 1 Berat 11 ZAZ … "Sinje" 1108 "\"3… 419
#> 4 Berat 1 Kucove 13 ZAZ … "Lumas" 1104 "\"3… 237
#> 5 Berat 1 Kucove 13 ZAZ … "Kucove" 1201 "\"3… 562
#> 6 Berat 1 Skrapar 17 ZAZ … "Corovode" 1303 "\"3… 829
#> 7 Berat 1 Berat 11 ZAZ … "Roshnik" 1107 "\"3… 410
#> 8 Berat 1 Ura Vajg… 19 ZAZ … "Ura Vajgurore" 1110 "\"3… 708
#> 9 Berat 1 Kucove 13 ZAZ … "Perondi" 1203 "\"3… 835
#> 10 Berat 1 Kucove 13 ZAZ … "Kucove" 1201 "\"3… 907
#> # ℹ 385 more rows
#> # ℹ 36 more variables: meshkuj <int>, femra <int>, totalSeats <int>,
#> # vendndodhja <fct>, ambienti <fct>, totalVoters <int>, femVoters <int>,
#> # maleVoters <int>, unusedBallots <int>, damagedBallots <int>,
#> # ballotsCast <int>, invalidVotes <int>, validVotes <int>, lsi <int>,
#> # ps <int>, pkd <int>, sfida <int>, pr <int>, pd <int>, pbdksh <int>,
#> # adk <int>, psd <int>, ad <int>, frd <int>, pds <int>, pdiu <int>, …Identify number of missing points by strata between sample and collected data