Emerging Hot Spot Analysis

Emerging Hot Spot Analysis identifies trends in spatial clustering over a period of time. Emerging hot spot analysis combines the Getis-Ord Gi* statistic with the Mann-Kendall trend test to determine if there is a temporal trend associated with local clustering of hot and cold spots.

Usage

emerging_hotspot_analysis(
  x,
  .var,
  k = 1,
  include_gi = FALSE,
  nb_col = NULL,
  wt_col = NULL,
  nsim = 199,
  threshold = 0.01,
  ...
)

Arguments

x: a spacetime object and must be a spacetime cube see details for more.
.var: a numeric vector in the spacetime cube with no missing values.
k: default 1. The number of time lags to include in the neighborhood for calculating the local Gi*. See details for more.
include_gi: default FALSE. If TRUE, includes the local Gi* calculations in the attribute gi_star.
nb_col: Optional. Default NULL. The name of the column in the geometry context of x containing spatial neighbors. If NULL, Queen's contiguity neighbors are identified.
wt_col: Optional. Default NULL. The name of the column in the geometry context of x containing spatial weights. If NULL, row standardized weights are used.
nsim: default 199. The number of simulations to run in calculating the simulated p-value for the local Gi*.
threshold: default 0.01. The significance threshold to use.
...: unused.

Value

Returns a data.frame.

Details

How Emerging Hot Spot Analysis Works

Emerging Hot Spot Analysis is a somewhat simple process. It works by first calculating the Gi* statistic for each location in each time period (time-slice). Next, for each location across all time-periods, the Mann-Kendall trend test is done to identify any temporal trend in Gi* values over all time periods. Additionally, each location is classified into one of seventeen categories based on ESRI's emerging hot spot classification criteria.

The Mann-Kendall trend test is done using Kendall::MannKendall(). Kendall is not installed with sfdep and should be installed prior to use.

Using your own neighbors and weights

If you would like to use your own neighbors and weights, they must be created in the geometry context of a spacetime object. The arguments nb_col and wt_col must both be populated in order to use your own neighbor and weights definitions.

Time lagged neighbors

In addition to identifying neighbors in space, emerging hotspot analysis also incorporates the same observations from k periods ago-called a time lag. If the time lag k is 1 and the unit of time is month, the neighbors for the calculation of Gi* would include the spatial neighbors' values at time t and the same spatial neighbors' values at time t-1. If k = 2, it would include t, t-1, and t-2.

Missing values

Presently, there is no method of missing value handling. If there are missing values, the emerging hot spot analysis will fail. Be sure to fill or omit time-slices with missing values prior to using emerging hot spot analysis.

Examples

df_fp <- system.file("extdata", "bos-ecometric.csv", package = "sfdep")
geo_fp <- system.file("extdata", "bos-ecometric.geojson", package = "sfdep")

# read in data
df <- readr::read_csv(df_fp, col_types = "ccidD")
geo <- sf::read_sf(geo_fp)

# Create spacetime object called `bos`
bos <- spacetime(df, geo,
                 .loc_col = ".region_id",
                 .time_col = "time_period")


# conduct EHSA
ehsa <- emerging_hotspot_analysis(
  x = bos,
  .var = "value",
  k = 1,
  nsim = 9
)

ehsa
#> # A tibble: 168 × 4
#>    location       tau p_value classification     
#>    <chr>        <dbl>   <dbl> <chr>              
#>  1 25025010405  0.111  0.721  no pattern detected
#>  2 25025010404 -0.333  0.210  no pattern detected
#>  3 25025010801 -0.200  0.474  no pattern detected
#>  4 25025010702 -0.600  0.0200 no pattern detected
#>  5 25025010204 -0.467  0.0736 no pattern detected
#>  6 25025010802 -0.333  0.210  no pattern detected
#>  7 25025010104 -0.333  0.210  no pattern detected
#>  8 25025000703 -0.600  0.0200 no pattern detected
#>  9 25025000504 -0.333  0.210  no pattern detected
#> 10 25025000704 -0.422  0.107  no pattern detected
#> # … with 158 more rows
#> # ℹ Use `print(n = ...)` to see more rows