# Using BCPA on a one-dimensional variable

#### November 14, 2018

The BCPA was originally formulated to analyze irregular movement data collected on marine mammals, but in essence it simply reduced movement data (X-Y-Time) to a univariate time-series. There are - in my opinion - better (i.e. more informative and more robust) tools for dealing with movement data specifically, (e.g. at https://github.com/EliGurarie/smoove), but the BCPA might still be useful for irregular univariate time series. An example (again from marine mammals) is depth data. A recent update to BCPA makes this analysis somewhat smoother. Here is an example on simulated data.

Note - to date this is available only on the GitHub version of BCPA, i.e. the first step is:

require(devtools)
install_github("EliGurarie/bcpa")

The code for this example can also be found in the help file for the WindowSweep() function.

# Analysis

## Depth data simulation

Load bcpa, and a few other handy packages:

require(magrittr)
require(lubridate)
require(bcpa)

We simulate some data with four phases / three change points: surface to medium to deep to surface, that occur at fixed times.

n.obs <- 100
time = (Sys.time() - dhours(runif(n.obs, 0, n.obs))) %>% sort

d1 <- 50; d2 <- 100
t1 <- 25; t2 <- 65; t3 <- 85
sd1 <- 1; sd2 <- 5; sd3 <- 10

dtime <- difftime(time, min(time), units="hours") %>% as.numeric
phases <- cut(dtime, c(-1, t1, t2, t3, 200), labels = c("P1","P2","P3","P4"))
means <- c(0,d1,d2,0)[phases]
sds <- c(sd1,sd2,sd3,sd1)[phases]
depth <- rnorm(n.obs,means, sds)
# make all depths positive!
depth <- abs(depth)
mydata <- data.frame(time, depth)

The structure of the data is very simple:

head(mydata)
##                  time      depth
## 1 2022-05-23 08:36:42 0.32192527
## 2 2022-05-23 09:13:09 0.78383894
## 3 2022-05-23 09:40:42 1.57572752
## 4 2022-05-23 10:08:49 0.64289931
## 5 2022-05-23 10:24:15 0.08976065
## 6 2022-05-23 11:44:36 0.27655075

Plot simulated depth data

with(mydata, plot(time, depth, type = "o"))

Perform the window sweep. Note that you specify the response variable (depth) and the time variable (time):

depth.ws <- WindowSweep(mydata, variable = "depth", time.var = "time", windowsize = 25, windowstep = 1, progress=FALSE)

Here are some plots and the summary of the change point analysis:

plot(depth.ws, ylab = "Depth (m)")

plot(depth.ws, type = "flat", cluster = 8, ylab = "Depth (m)")

ChangePointSummary(depth.ws, cluster = 8)
## $breaks ## X1 middle size modelmode middle.POSIX ## 1 1 25.46224 27 4 2022-05-24 09:58:01 ## 2 2 62.47157 18 4 2022-05-25 22:46:30 ## 3 3 84.46078 12 4 2022-05-26 20:34:54 ## ##$phases
##         t.cut    mu.hat     s.hat      rho.hat       t0       t1 interval
## 1   (-1,25.5]  3.681865 11.256694 1.4733085237 -1.00000 25.46224 26.46224
## 2 (25.5,62.5] 50.063357  4.367361 0.0005596285 25.46224 62.47157 37.00933
## 3 (62.5,84.5] 91.941686 17.048889 0.0883214493 62.47157 84.46078 21.98921
## 4 (84.5,98.9]  6.434163 22.852427 0.0613947945 84.46078 98.86528 14.40450

This is a pretty artificial example, but it works well.