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Calculate camera-days required to achieve a target CV

Source: R/power-sample-size.R
creel_n_camera.Rd

Uses the stratified sample size formula from Cochran (1977) to determine how many camera-days are needed to achieve a target coefficient of variation on the camera-effort estimate, given pilot mean and variance estimates per day-type stratum.

Usage

creel_n_camera(cv_target, N_h, ybar_h, s2_h)

Arguments

cv_target

Numeric scalar. Target coefficient of variation for the camera-effort estimate (e.g., 0.20 for 20 percent). Must be in (0, 1].

N_h

Named numeric vector. Total available days per stratum (e.g., c(weekday = 65, weekend = 28)). Values must be >= 1.

ybar_h

Numeric vector of same length as N_h. Pilot mean camera count per day per stratum. Values must be >= 0.

s2_h

Numeric vector of same length as N_h. Pilot variance of camera counts per day per stratum. Values must be >= 0.

Value

A named integer vector. Elements named after strata in N_h give the camera-days required per stratum; element "total" gives the overall sample size before proportional allocation.

Details

Implements Cochran (1977) equation 5.25 under proportional allocation. The finite-population correction (FPC) factor is intentionally omitted (standard practice for pre-season planning where the goal is to determine how many days to deploy cameras, not to assess precision of a completed survey).

The per-stratum sample sizes n_h are computed from the total n_total under proportional allocation: n_h = ceiling(n_total * N_h / sum(N_h)). Because each stratum is ceiling-ed independently, sum(n_h) may exceed n_total.

Minimum camera-day check: After computing n_h, the function applies empirical minimums from Feltz-Middaugh (2025). Stratum names are matched case-insensitively and partially:

  • Names containing "weekday" require a minimum of 12 camera-days.

  • Names containing "weekend" require a minimum of 7 camera-days.

  • Names matching neither pattern trigger a generic advisory (no numeric floor — consult Feltz-Middaugh 2025 for the appropriate minimum).

When any stratum is below threshold (or is unclassified), a single combined cli_warn() is emitted listing all affected strata with their computed n and minimum side-by-side. No warning fires if all classified strata meet or exceed their minimums and there are no unclassified strata.

References

Cochran, W.G. 1977. Sampling Techniques, 3rd ed. Wiley, New York.

Feltz, C.J. and Middaugh, C.R. 2025. Minimum camera-day requirements for reliable creel-camera effort estimation. North American Journal of Fisheries Management. (in press)

See also

creel_n_effort() for the equivalent function for angler-contact sampling days.

Other "Planning & Sample Size": audit_strata(), compare_designs(), creel_n_cpue(), creel_n_effort(), creel_power(), cv_from_n(), power_creel(), reallocate_strata(), simulate_strata_collapse()

Examples

# Two-stratum weekday/weekend example
creel_n_camera(
  cv_target = 0.20,
  N_h = c(weekday = 65, weekend = 28),
  ybar_h = c(15, 20),
  s2_h = c(625, 900)
)
#> weekday weekend   total 
#>      27      12      38