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Convert a count time series to fractional change relative to a baseline period

Source: R/analysis_functions.R
calc_percent_change.Rd

For each group, computes the mean of count_var over a baseline period of n_baseline_periods consecutive time steps starting at start_date (or the earliest date if start_date is NULL). Each observation is then expressed as a fractional change relative to that baseline mean:

Usage

calc_percent_change(
  dat,
  date_var = "date",
  grp_var,
  count_var,
  n_baseline_periods = 7,
  start_date = NULL
)

Arguments

dat

A data frame containing a count column, a date column, and a grouping column.

date_var

Character string giving the name of the date column (class Date). Default is "date".

grp_var

Character string giving the name of the grouping column. The baseline mean is computed separately per group.

count_var

Character string giving the name of the count column.

n_baseline_periods

Integer. Number of consecutive time steps used to compute the baseline mean. For daily data, 7 gives a one-week baseline. Default is 7.

start_date

Start date of the baseline period. Accepts a Date object or a character string in "YYYY-MM-DD" format (e.g. "2020-01-13"). If NULL (default), the earliest date across the combined dataset is used as the baseline start.

Value

The input data frame with one additional numeric column, perc_change, giving each observation as a fractional change relative to the group-specific baseline mean (0 = no change from baseline, -1 = zero counts, positive values = above baseline).

Details

$$\texttt{perc\_change} = \frac{\texttt{count} - \texttt{baseline mean}}{\texttt{baseline mean}}$$

A value of 0 indicates no change from baseline; -0.5 indicates a 50% decrease; 1.0 indicates a doubling. Originally developed for population mobility data but applicable to any non-negative count series.

Examples

if (FALSE) { # \dontrun{
dat_pct <- calc_percent_change(
  dat                = mobility_data,
  date_var           = "date",
  grp_var            = "region",
  count_var          = "trips",
  n_baseline_periods = 7,
  start_date         = "2020-01-13"
)
} # }